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This CL adds chrome://notifications-internals, which can be served as
a place to debug notification related features.

Bug: 963289
Change-Id: Ib13d99c5acd9e6b5025c74d230ade98e1e2b6c47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1659690
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670209}