Refactors event dispatching of NativeViewport into its own interface

Refactors event dispatching of NativeViewport into its own interface

This way the WindowManager can handle the event directly. New flow is
for the ViewManager to request the NativeViewportEventDispatcher from
the WindowManager and connect said NativeViewportEventDispatcher to
the NativeViewport.

BUG=none
TEST=none
R=ben@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/c947eccd5acb557128d9af9f6212a8524e910443

[Ben Goodger (Google), 2014-10-28 22:58:51]

lgtm

[sky, 2014-10-28 23:49:59]

Committed patchset #3 (id:40001) manually as
c947eccd5acb557128d9af9f6212a8524e910443 (presubmit successful).