Add initial CC support for scroll-blocks-on

cc/input/input_handler.h

Index: cc/input/input_handler.h
diff --git a/cc/input/input_handler.h b/cc/input/input_handler.h
index 020862726a3c65c859dc835e046dbf4800290e1d..d899a85156e4b63160f3d946b6a65398b2a3aff8 100644
--- a/cc/input/input_handler.h
+++ b/cc/input/input_handler.h
@@ -143,7 +143,10 @@ class CC_EXPORT InputHandler {
                                            ScrollInputType type) = 0;
 
   virtual bool HaveWheelEventHandlersAt(const gfx::Point& viewport_point) = 0;

[jdduke (slow), 2015/02/03 16:43:34]

Now that "blocks on" is the phrasing used for both wheel events and touch
events, is there a reason to have different names for the two queries
(HaveWheelEventHandlersAt vs DoTouchEventsBlockScrollAt)? I guess the semantics
are slightly different, but I wonder if there's a consistent naming scheme here
that communicates just the necessary bits, e.g.,

|ShouldForwardTouchToMain|
|ShouldForwardWheelToMain|

or something like that? Not a huge deal though.

[Rick Byers, 2015/02/05 04:35:53]

Yeah I considered changing this but the semantics are different.  In particular
HaveWheelEventHandlersAt returns ONLY if there are wheel event handlers
(scroll-blocks-on handling is handled outside).  DoTouchEventBlockScrollAt looks
at both touch handler rects and the scroll-blocks-on mode.  The reason for the
difference is just the different places these are used -
HaveWheelEventHandlersAt is part of the scrolling system (which already looks at
scroll-blocks-on), but DoTouchEventsBlockScroll is used more in isolation to
decide when to forward touch events to blink.  So I think the asymmetry reflects
a real difference - it's now misleading to think of these as the same thing.

But I only spent a few minutes looking for ways to refactor things to be more
symmetric.  I'm open to suggestions.

-  virtual bool HaveTouchEventHandlersAt(const gfx::Point& viewport_point) = 0;
+
+  // Whether the page should be given the opportunity to suppress scrolling by
+  // consuming touch events that started at |viewport_point|.
+  virtual bool DoTouchEventsBlockScrollAt(const gfx::Point& viewport_point) = 0;
 
   // Calling CreateLatencyInfoSwapPromiseMonitor() to get a scoped
   // LatencyInfoSwapPromiseMonitor. During the life time of the