[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 b38154f0d992c0e9a9ff441bd65c469c64684227..86bf4a2a515b35ddcf578a412412666c306dce97 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::BeginFrameNotExpectedSoon() {
+}
+
 void FakeRendererScheduler::DidCommitFrameToCompositor() {
 }