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

Review URL: https://codereview.chromium.org/685013002