Tighter sync and faster ACK on SwapBuffers/PostSubBuffers

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index 9ad531afc40668b06356b4356d9a413134ab6893..7e852a1fd8a9695bdd570cbf9a9127d1bf44a57c 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -131,7 +131,10 @@ Compositor::Compositor(CompositorDelegate* delegate,
       widget_(widget),
       root_web_layer_(WebKit::WebLayer::create()),
       swap_posted_(false),
-      device_scale_factor_(0.0f) {
+      device_scale_factor_(0.0f),
+      last_started_id_(0),
+      last_will_end_id_(0),
+      last_ended_id_(0) {
   WebKit::WebLayerTreeView::Settings settings;
   CommandLine* command_line = CommandLine::ForCurrentProcess();
   settings.showFPSCounter =
@@ -204,10 +207,18 @@ void Compositor::Draw(bool force_clear) {
   if (!root_layer_)
     return;
 
+  last_started_id_++;
+
   // TODO(nduca): Temporary while compositor calls
   // compositeImmediately() directly.
   layout();
   host_.composite();
+
+  last_will_end_id_++;
+  FOR_EACH_OBSERVER(CompositorObserver,
+                    observer_list_,
+                    OnCompositingWillEnd(this));
+
   if (!g_compositor_thread && !swap_posted_)
     NotifyEnd();
 }
@@ -347,6 +358,7 @@ void Compositor::SwizzleRGBAToBGRAAndFlip(unsigned char* pixels,
 }
 
 void Compositor::NotifyEnd() {
+  last_ended_id_++;
   FOR_EACH_OBSERVER(CompositorObserver,
                     observer_list_,
                     OnCompositingEnded(this));