Extract AccessibilityBridge::kRootNodeId (flutter/engine#29856)

The ID of the root semantics node in Flutter's semantics tree is always
0. Since we'll be adding support for Windows, extract this constant to a
common location.

engine/src/flutter/shell/platform/common/accessibility_bridge.h

@@ -100,6 +100,7 @@ class AccessibilityBridge
     virtual std::shared_ptr<FlutterPlatformNodeDelegate>
     CreateFlutterPlatformNodeDelegate() = 0;
   };
+
   //-----------------------------------------------------------------------------
   /// @brief      Creates a new instance of a accessibility bridge.
   ///
@@ -110,6 +111,11 @@ class AccessibilityBridge
       std::unique_ptr<AccessibilityBridgeDelegate> delegate);
   ~AccessibilityBridge();
 
+  //-----------------------------------------------------------------------------
+  /// @brief      The ID of the root node in the accessibility tree. In Flutter,
+  //              this is always 0.
+  static constexpr int32_t kRootNodeId = 0;
+
   //------------------------------------------------------------------------------
   /// @brief      Adds a semantics node update to the pending semantics update.
   ///             Calling this method alone will NOT update the semantics tree.

engine/src/flutter/shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMacDelegate.mm

@@ -12,8 +12,6 @@
 
 namespace flutter {
 
-inline constexpr int32_t kRootNode = 0;
-
 // Native mac notifications fired. These notifications are not publicly documented.
 static NSString* const AccessibilityLoadCompleteNotification = @"AXLoadComplete";
 static NSString* const AccessibilityInvalidStatusChangedNotification = @"AXInvalidStatusChanged";
@@ -187,9 +185,10 @@ AccessibilityBridgeMacDelegate::MacOSEventsFromAXEvent(ui::AXEventGenerator::Eve
         if (ax_node.data().HasState(ax::mojom::State::kEditable)) {
           events.push_back({
               .name = NSAccessibilityValueChangedNotification,
-              .target = bridge->GetFlutterPlatformNodeDelegateFromID(kRootNode)
-                            .lock()
-                            ->GetNativeViewAccessible(),
+              .target =
+                  bridge->GetFlutterPlatformNodeDelegateFromID(AccessibilityBridge::kRootNodeId)
+                      .lock()
+                      ->GetNativeViewAccessible(),
               .user_info = nil,
           });
         }