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

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index f4780789394c447a1b3a56cfb5925ef325dec6a1..56bca4f8028dd8b8cb4d045e66f3a20a848102e8 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -233,6 +233,10 @@ void LayerTreeHost::DidBeginMainFrame() {
   client_->DidBeginMainFrame();
 }
 
+void LayerTreeHost::BeginMainFrameNotExpectedSoon() {
+  client_->BeginMainFrameNotExpectedSoon();
+}
+
 void LayerTreeHost::BeginMainFrame(const BeginFrameArgs& args) {
   inside_begin_main_frame_ = true;
   client_->BeginMainFrame(args);