Add missing dependency to the build

We've had a number of reports of a missing dependency in the build. I managed
to reproduce the issue locally today and this patch fixed the issue I was
seeing. Hopefully it fixes the issues other folks were seeing too.

The problem was that the C++ code we generate from IDL files depended on
various event classes in core, which in turn depended on the generated event
names files. This CL ensures that the event names files are created before we
try to build the C++ code generated from the IDL files.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1178173004.

engine/bindings/BUILD.gn

@@ -384,6 +384,7 @@ source_set("generated_bindings") {
     "//mojo/services/navigation/public/interfaces",
     "//mojo/services/view_manager/public/cpp",
     "//skia",
+    "//sky/engine/core:core_names",
     "//sky/engine/wtf",
     "//third_party/angle:translator",
     "//third_party/iccjpeg",