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This method was used in the following places:
(1) TabScrubber, where the code did nothing.
(2) BrowserNonClientFrameViewMus, to handle the tabstrip being deleted before
    the frame.
(3) GlassBrowserFrameView, to NOTREACHED() because the tabstrip should never be
    deleted before the frame.
(4) A test that the call was fired.

Because of (3), I assume (2) is actually dead code.  Since that's the only
functional use, it seems like this whole method can disappear.

BUG=none
TEST=none

Change-Id: I5945ccd019741eff09df286bf5ae1bfa82337761
Reviewed-on: https://chromium-review.googlesource.com/1039004
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555480}