Implement compositor thread VSync aligned event queue

Add compositor thread event queue to |InputHandlerProxy|, queues
|GestureScroll/Pinch| and dispatch on |WillBeginImplFrame()| signal
from |LayerTreeHost|.

Currently behind a feature flag and is disabled.

Future Task:
Coalesce events across start/end boundaries.

Design Doc:
https://docs.google.com/document/d/1SUPPJkqxEYMRso3Jw1wZnqD3g1dmif0ic-x5aQkpHBo/edit?usp=sharing

BUG=625689
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2429953002
Cr-Commit-Position: refs/heads/master@{#433275}
diff --git a/cc/input/input_handler.h b/cc/input/input_handler.h
index 6abb510..d60e9d4 100644
--- a/cc/input/input_handler.h
+++ b/cc/input/input_handler.h
@@ -61,6 +61,7 @@
       float page_scale_factor,
       float min_page_scale_factor,
       float max_page_scale_factor) = 0;
+  virtual void DeliverInputForBeginFrame() = 0;
 
  protected:
   InputHandlerClient() {}