[Android Notification Channels] Record when sites channel is used

- This is a precursor to removing the generic 'Sites' notification
channel - now that we should have a specific site channel for each
origin with notification permission, the sites channel should never be
used, but adding UMA to verify this.

Bug: 802380
Change-Id: I911b569dad291a4a9f01530f1185adc4dbcaa41e
Reviewed-on: https://chromium-review.googlesource.com/1159020
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579912}
3 files changed