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Replace old API with new API. This CL is a refactor
and has no intended behavior change.

To keep TabStrip UI consistent with model, make tabstrip UI the
first observer.

Bug: 842194
Change-Id: Icba9481d41ba32a7910593198cebd313ad531e5d
Reviewed-on: https://chromium-review.googlesource.com/c/1249461
Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604497}