Auto-fetch: Implement system notification

When auto-fetch pages are fetched, the user is now notified through a system
notification. If the notification is tapped, the page is opened in Chrome.

Bug: 883486
Change-Id: I0c15cf2832f16f21992ecb74b5f223fd0824daf9
Reviewed-on: https://chromium-review.googlesource.com/c/1332392
Commit-Queue: Dan H <harringtond@google.com>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621239}
29 files changed