Clean up Cast notification code
This CL:
* removes the code for recreating the notification following a Chrome
crash. This has never worked correctly, and is (no longer?) required
by the Chromecast UI guidelines.
* changes the behavior when the Chrome task is swiped away to stop
casting immediately (and remove the notification)
* refactors the code to move most of the functionality into the
notification service.
BUG=515086
BUG=515055
BUG=467700
BUG=454431
BUG=449462
Review URL: https://codereview.chromium.org/
1214833005
Cr-Commit-Position: refs/heads/master@{#345315}