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This CL removes the HEADS_UP_NOTIFICATION and QUIET_NOTIFICATION
user-facing UI surfaces, namely the "chrome://flags" screen and
the chrome notification channels settings screen.

This ensure that either by user-intervention or auto-embargo
logic, only the mini-infobar will be selected when a quiet
UI needs to be shown.

Follow-up CL will remove/refactor all the code that has now
become useless, related to the ability to display the
permission request prompt as a notification.

Bug: 1029718
Change-Id: Ib01e50209184c81abb53770e21a3deda9a2228ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1946469
Commit-Queue: Andy Paicu <andypaicu@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720509}