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This CL removes the dependency on ActivityTab{Provider/TabObserver} in
toolbar/ package. They are replaced with a new CurrentTabObserver that
works as a stop-gap solution.

Bug: 1127732
Change-Id: I69e4272d38e26e70e62c31a6400859832dd70a2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505297
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826541}