views: Dispatch mouse events using EventDispatch interface too.

views: Dispatch mouse events using EventDispatch interface too.

Some of the notable changes:
 * View::OnMouseEntered and OnMouseExited functions will now be triggered with
   the correct event-types (i.e. ET_MOUSE_ENTERED and ET_MOUSE_EXITED). These
   used to be triggered with ET_MOUSE_MOVED before.
 * Introduce View::GetDragInfo(). This allows a View to reach into the
   RootView's drag_info and update it as necessary for dragging.
 * View override of OnMouseEvent() triggers the appropriate callback (e.g.
   OnMousePressed, OnMouseRelease etc.), instead of the RootView directly
   calling into these methods.

BUG=173235
(Previously committed in r182804, and reverted in r182864 because it broke some tests)

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=183039

[sadrul, 2013-02-11 20:14:12]

This is the last remaining piece for views to use EventDispatcher to dispatch
events (https://codereview.chromium.org/12207092/ takes care of the rest of the
event types).

[Ben Goodger (Google), 2013-02-14 23:28:17]

lgtm