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This CL implements a new system for managing custom Views for Tab.
The TabViewManager class acts as the entry point for all users who want
to display a View on top of Tab's Content view.

TabViewManager knows the priority of all its users and always displays
the TabViewProvider with the highest priority.

Internal design doc: http://go/tab-view-clients

Bug: 1073948
Change-Id: I5d5d0384b512d6722dd0357295de5542b9b77306
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159248Reviewed-by: Theresa  <twellington@chromium.org>
Reviewed-by: Patrick Noland <pnoland@chromium.org>
Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Commit-Queue: Mehran Mahmoudi <mahmoudi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763351}