[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 b7ed37373c0a1e68cdaabc57e27ac82edcf3676e..0acb6878b00ce2d125bfb481479cd3916c526fd1 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -301,6 +301,10 @@ void Compositor::BeginMainFrame(const cc::BeginFrameArgs& args) {
     host_->SetNeedsAnimate();
 }
 
+void Compositor::BeginMainFrameNoLongerRequired() {
+  // TODO(rmcilroy): Should we do anything here?
+}
+
 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.