Refactor compositor event handling path to be callback-based

ui/events/blink/input_handler_proxy_client.h

Index: ui/events/blink/input_handler_proxy_client.h
diff --git a/ui/events/blink/input_handler_proxy_client.h b/ui/events/blink/input_handler_proxy_client.h
index efc24ed6d17d4a83b8879422c96db83481f7d89a..b601ea8f2f0f0f61eef9e5fe787aa4ce441efe61 100644
--- a/ui/events/blink/input_handler_proxy_client.h
+++ b/ui/events/blink/input_handler_proxy_client.h
@@ -32,6 +32,10 @@ class InputHandlerProxyClient {
       const blink::WebFloatPoint& velocity,
       const blink::WebSize& cumulative_scroll) = 0;
 
+  // Overscroll triggered by |HandleInputEvent/WithLatencyInfo| will response to

[tdresser, 2016/09/01 16:58:45]

This sentence doesn't quite make sense.

"|HandleInputEvent/WithLatencyInfo| will respond to overscroll by calling the
passed in callback." perhaps?

[chongz, 2016/09/01 20:13:13]

Done.

+  // passed-in callback. (e.g. Usually
+  // |InputHandlerManager::DidHandleInputEventAndOverscroll|)
+  // Otherwise |DidOverscroll| will be fired.
   virtual void DidOverscroll(
       const gfx::Vector2dF& accumulated_overscroll,
       const gfx::Vector2dF& latest_overscroll_delta,