compositor-worker: cc->blink mutation plumbing

cc/trees/proxy_main.cc

Index: cc/trees/proxy_main.cc
diff --git a/cc/trees/proxy_main.cc b/cc/trees/proxy_main.cc
index 3a98ff59d49b43b0b79a96cc4f672d926a3e0d32..eae3ad4f3e27809f8d4115883268c6f887ae39ff 100644
--- a/cc/trees/proxy_main.cc
+++ b/cc/trees/proxy_main.cc
@@ -187,6 +187,11 @@ void ProxyMain::BeginMainFrame(
   layer_tree_host_->ApplyScrollAndScale(
       begin_main_frame_state->scroll_info.get());
 
+  if (begin_main_frame_state->begin_frame_callbacks) {
+    for (auto& callback : *begin_main_frame_state->begin_frame_callbacks)
+      callback.Run();
+  }
+
   layer_tree_host_->WillBeginMainFrame();
 
   layer_tree_host_->BeginMainFrame(begin_main_frame_state->begin_frame_args);