gpu: Add flag for enabling asynchronous worker context.

content/test/fake_compositor_dependencies.cc

Index: content/test/fake_compositor_dependencies.cc
diff --git a/content/test/fake_compositor_dependencies.cc b/content/test/fake_compositor_dependencies.cc
index f379dc930f98172bfbb1bbb80934f093186c7857..532f4dabdda2f40ac10339644373245b3a7b56a0 100644
--- a/content/test/fake_compositor_dependencies.cc
+++ b/content/test/fake_compositor_dependencies.cc
@@ -29,6 +29,10 @@ bool FakeCompositorDependencies::IsGpuRasterizationEnabled() {
   return false;
 }
 
+bool FakeCompositorDependencies::IsAsyncWorkerContextEnabled() {
+  return false;
+}
+
 int FakeCompositorDependencies::GetGpuRasterizationMSAASampleCount() {
   return 0;
 }