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This change is behind the kReadLater feature flag and uses styles shared
with the Tab Search feature. The delete button removes entries from the
reading list model and the mark as read/unread button updates the
status. Both buttons will trigger an update to the menu items so changes
are seen as they are triggered.

Bug: 1109316
Change-Id: Iaa093427c4f9be78901833eca3fd1dabec8ec1b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422246
Commit-Queue: Caroline Rising <corising@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810050}