Fix ObserverList assertions in TabStripTest

BUG=1230198

Change-Id: I16187efe072fdcf06b4c7030bf51f9e7ee437d72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3072885


Commit-Queue: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Pavel Yatsuk <pavely@chromium.org>
Auto-Submit: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908684}