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Reverts cl/2414613 and adds the ability to trigger the Discover feed
IPH bubble manually, rather than at the same time as the other bubbles.

Stores a reference to the Discover feed header menu button in the
ContentSuggestionsCoordinator. This allows for the menu button to be
accessed whenever it is needed, in order to create a named guide for
the IPH to work. Also, this allows for the feed alert menu to access
the button more easily.

Change-Id: I7cd50c922af8e2ace6914e680cce7f494e325bb6
Bug: 1139320, 1131571
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507386
Commit-Queue: Adam Trudeau-Arcaro <adamta@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822785}