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This adds a Tab.setData() and Tab.getData() API which allows setting
arbitrary embedder specific data on a tab. This data will be
saved/restored along with other browser data.

Bug: 1079313
Change-Id: Iccc0f17e647c4252768e2b1e69e2c87d9f695de1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2204540Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770274}