Message Center Touch Feedback
Implement touch feedback for MessageView. The background will draw as active in response to touch interactions. Touch downs will turn the view active, and it will remain active until a user releases the touch.
Unit tests were not added to MessageCenterViewUnittest as it is a base gtest with no gesture recognizer. Manually injecting touch events is not supported by view::Views. Manually injecting the gestures would be eqivalent of trivial a==a tests.
ash_switches flag for feedback has not been added due to ui/ being independant of ash/
TEST=Manual test on device. Open message center, the messages should become active upon pressing down, and should lose the active state upon release.
BUG=398398
Review URL: https://codereview.chromium.org/
558673002
Cr-Commit-Position: refs/heads/master@{#297700}