Drop frontbuffers with ui-use-gpu-process, synchronized with browser, decoupled from backbuffer dro…

ui/compositor/compositor.h

Index: ui/compositor/compositor.h
diff --git a/ui/compositor/compositor.h b/ui/compositor/compositor.h
index 33f6d6b6c3a9ef78348b56518b734b91809b4d01..6efcf700d540e28f65d0b7f497874fd33d7b628b 100644
--- a/ui/compositor/compositor.h
+++ b/ui/compositor/compositor.h
@@ -191,6 +191,8 @@ class COMPOSITOR_EXPORT Compositor
   // and the OnCompositingEnded.
   bool DrawPending() const { return swap_posted_; }
 
+  bool CurrentlyCompositing() const { return currently_compositing_; }
+
   // Internal functions, called back by command-buffer contexts on swap buffer
   // events.
 
@@ -241,6 +243,9 @@ class COMPOSITOR_EXPORT Compositor
   // for completion.
   bool swap_posted_;
 
+  // This is true between OnCompositingStarted and OnCompositingEnded
+  bool currently_compositing_;
+
   // The device scale factor of the monitor that this compositor is compositing
   // layers on.
   float device_scale_factor_;