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Added metadata to Tab, TabSlotView

https://docs.google.com/document/d/1FXacwqJjW4ircJ19gsypRnLAWMYmM8sFTyDCpBH48jI/edit?usp=sharing

Bug: 938501
Change-Id: I659ca7eea66d8b50e0609d08b29cfc8efd3b8809
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429515
Commit-Queue: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810503}