[web] Check for cached browsers on LUCI (flutter/engine#37721)

engine/src/flutter/.ci.yaml

@@ -273,6 +273,8 @@ targets:
         {"download_emsdk": true}
       dependencies: >-
         [
+          {"dependency": "chrome_and_driver", "version": "version:107.0"},
+          {"dependency": "firefox", "version": "version:83.0"},
           {"dependency": "goldctl", "version": "git_revision:3a77d0b12c697a840ca0c7705208e8622dc94603"}
         ]
     timeout: 60
@@ -501,6 +503,10 @@ targets:
       gclient_variables: >-
         {"download_emsdk": true}
       gcs_goldens_bucket: flutter_logs
+      dependencies: >-
+        [
+          {"dependency": "chrome_and_driver", "version": "version:107.0"}
+        ]
     timeout: 60
     runIf:
       - DEPS

engine/src/flutter/lib/web_ui/README.md

@@ -153,6 +153,20 @@ We test with Firefox on LUCI in the Linux Web Engine builder. The process for
 rolling Firefox is even easier than Chromium. Simply update `browser_lock.yaml`
 with the latest version of Firefox, and run `browser_roller.dart`.
 
+#### .ci.yaml
+
+After rolling Chrome and/or Firefox, also update the CI dependencies in
+`.ci.yaml` to make use of the new versions. The lines look like
+
+```yaml
+      dependencies: >-
+        [
+          {"dependency": "chrome_and_driver", "version": "version:107.0"},
+          {"dependency": "firefox", "version": "version:83.0"},
+          {"dependency": "goldctl", "version": "git_revision:3a77d0b12c697a840ca0c7705208e8622dc94603"}
+        ]
+```
+
 ##### **browser_roller.dart**
 
 The script has the following command-line options:

engine/src/flutter/lib/web_ui/dev/chrome_installer.dart

@@ -15,6 +15,8 @@ import 'common.dart';
 import 'environment.dart';
 import 'exceptions.dart';
 
+const String _chromeExecutableVar = 'CHROME_EXECUTABLE';
+
 /// Returns the installation of Chrome, installing it if necessary.
 ///
 /// If [requestedVersion] is null, uses the version specified on the
@@ -33,6 +35,18 @@ Future<BrowserInstallation> getOrInstallChrome(
 }) async {
   infoLog ??= io.stdout;
 
+  // When running on LUCI, if we specify the "chrome_and_driver" dependency,
+  // then the bot will download Chrome from CIPD and place it in a cache and
+  // set the environment variable CHROME_EXECUTABLE.
+  if (io.Platform.environment.containsKey(_chromeExecutableVar)) {
+    infoLog.writeln('Using Chrome from $_chromeExecutableVar variable: '
+      '${io.Platform.environment[_chromeExecutableVar]}');
+    return BrowserInstallation(
+      version: 'cipd',
+      executable: io.Platform.environment[_chromeExecutableVar]!,
+    );
+  }
+
   if (requestedVersion == 'system') {
     return BrowserInstallation(
       version: 'system',

engine/src/flutter/lib/web_ui/dev/firefox.dart

@@ -22,7 +22,7 @@ class FirefoxEnvironment implements BrowserEnvironment {
 
   @override
   Future<Browser> launchBrowserInstance(Uri url, {bool debug = false}) async {
-    return Firefox(url, this, debug: debug);
+    return Firefox(url, _installation, debug: debug);
   }
 
   @override
@@ -56,8 +56,7 @@ class FirefoxEnvironment implements BrowserEnvironment {
 class Firefox extends Browser {
   /// Starts a new instance of Firefox open to the given [url], which may be a
   /// [Uri] or a [String].
-  factory Firefox(Uri url, FirefoxEnvironment firefoxEnvironment, {bool debug = false}) {
-    final BrowserInstallation installation = firefoxEnvironment._installation;
+  factory Firefox(Uri url, BrowserInstallation installation, {bool debug = false}) {
     final Completer<Uri> remoteDebuggerCompleter = Completer<Uri>.sync();
     return Firefox._(BrowserProcess(() async {
       // Using a profile on opening will prevent popups related to profiles.

engine/src/flutter/lib/web_ui/dev/firefox_installer.dart

@@ -11,6 +11,8 @@ import 'common.dart';
 import 'environment.dart';
 import 'exceptions.dart';
 
+const String _firefoxExecutableVar = 'FIREFOX_EXECUTABLE';
+
 /// Returns the installation of Firefox, installing it if necessary.
 ///
 /// If [requestedVersion] is null, uses the version specified on the
@@ -36,6 +38,18 @@ Future<BrowserInstallation> getOrInstallFirefox(
 
   infoLog ??= io.stdout;
 
+  // When running on LUCI, if we specify the "firefox" dependency, then the
+  // bot will download Firefox from CIPD and place it in a cache and set the
+  // environment variable FIREFOX_EXECUTABLE.
+  if (io.Platform.environment.containsKey(_firefoxExecutableVar)) {
+    infoLog.writeln('Using Firefox from $_firefoxExecutableVar variable: '
+      '${io.Platform.environment[_firefoxExecutableVar]}');
+    return BrowserInstallation(
+      version: 'cipd',
+      executable: io.Platform.environment[_firefoxExecutableVar]!,
+    );
+  }
+
   if (requestedVersion == 'system') {
     return BrowserInstallation(
       version: 'system',