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

content/renderer/scheduler/null_renderer_scheduler.cc

Index: content/renderer/scheduler/null_renderer_scheduler.cc
diff --git a/content/renderer/scheduler/null_renderer_scheduler.cc b/content/renderer/scheduler/null_renderer_scheduler.cc
index 4ac7bd47a68f931e9e664d4cf189c07ce2822ecc..00b5c47e6e9167446566a8c44ce9758aa42a0de0 100644
--- a/content/renderer/scheduler/null_renderer_scheduler.cc
+++ b/content/renderer/scheduler/null_renderer_scheduler.cc
@@ -68,6 +68,9 @@ NullRendererScheduler::IdleTaskRunner() {
 void NullRendererScheduler::WillBeginFrame(const cc::BeginFrameArgs& args) {
 }
 
+void NullRendererScheduler::BeginFrameNotExpectedSoon() {
+}
+
 void NullRendererScheduler::DidCommitFrameToCompositor() {
 }