Move event dispatch to view manager

Move event dispatch to view manager

This work is motivated by a couple of things:
. Proper event targetting needs to be closely tied with the tree of
  views and potentially needs to dispatch to multiple views.
. To decrease the number of apps an event goes through.
. To more narrowly focus the responsibilities of the window manager.

This is the same as https://codereview.chromium.org/1051763005/ , but
in chromium.

BUG=none
TEST=none
R=erg@chromium.org,ben@chromium.org
TBR=erg@chromium.org,ben@chromium.org

Review URL: https://codereview.chromium.org/1093373004

Cr-Commit-Position: refs/heads/master@{#326602}