Add command-line flag to force compositor framerate.

ui/compositor/compositor.h

Index: ui/compositor/compositor.h
diff --git a/ui/compositor/compositor.h b/ui/compositor/compositor.h
index 25c3e0c81652461a79833f9bf709b0460daf5816..403a1108700761d53e563d7cfd40a492a6c09104 100644
--- a/ui/compositor/compositor.h
+++ b/ui/compositor/compositor.h
@@ -402,6 +402,9 @@ class COMPOSITOR_EXPORT Compositor
   // Current vsync refresh rate per second.
   float refresh_rate_ = 0.f;
 
+  // If nonzero, this is the refresh rate forced from the command-line.
+  double forced_refresh_rate_ = 0.f;
+
   // A map from child id to parent id.
   std::unordered_set<cc::FrameSinkId, cc::FrameSinkIdHash> child_frame_sinks_;
   bool widget_valid_ = false;