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

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index bea2eb7f3c98f7725e7166edf4f7b3d6e8cebbe9..2dab0b9e2285d6fbe99677af40c652d34a5fafe9 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -303,6 +303,9 @@ void Compositor::BeginMainFrame(const cc::BeginFrameArgs& args) {
     host_->SetNeedsAnimate();
 }
 
+void Compositor::BeginMainFrameNotExpectedSoon() {
+}
+
 void Compositor::Layout() {
   // We're sending damage that will be addressed during this composite
   // cycle, so we don't need to schedule another composite to address it.