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This hooks up the existing web badging API to the UI so that a PWA
will show an attention badge if the PWA has called the web badging
API.

- When the flag "enable-desktop-pwas-attention-badging-cros" is set to
  "notifications-only", a badge will only show when a notification is
  showing.
- When the flag is set to "api-only" a badge will only show if the web
  badging api has a badge set.
- When the flag is set to "api-and-notifications" a badge will show
  either if a notification is showing or if the web badging api has a
  badge set. This is the default behaviour.

Bug: 1157273
Change-Id: Ic0275e5e2d023f13554ed6ca001665b69f49f58e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2589055
Commit-Queue: Louise Brett <loubrett@google.com>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837903}