[cc]: Add a BeginMainFrameNotExpectedSoon signal and route it to the RendererScheduler.

content/test/fake_renderer_scheduler.cc

Index: content/test/fake_renderer_scheduler.cc
diff --git a/content/test/fake_renderer_scheduler.cc b/content/test/fake_renderer_scheduler.cc
index 2e20b7f9b46c5dfc56d6fd0f2a659f7a10c61168..7394a39ffa84dcbdb18fac4f2e677822343a42ad 100644
--- a/content/test/fake_renderer_scheduler.cc
+++ b/content/test/fake_renderer_scheduler.cc
@@ -35,6 +35,9 @@ FakeRendererScheduler::IdleTaskRunner() {
 void FakeRendererScheduler::WillBeginFrame(const cc::BeginFrameArgs& args) {
 }
 
+void FakeRendererScheduler::BeginFrameNoLongerRequired() {
+}
+
 void FakeRendererScheduler::DidCommitFrameToCompositor() {
 }