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This patch refactors the single tab mode to be tracked by the tabstrip,
and enabled/disabled by BrowserNonClientFrameView when appropriate. The
tabstrip uses that mode flag to force the close-x for the active tab to
be invisible.

Bug: 841643
Change-Id: I0c8713e5190af935d665e7d10f984db8624b4ab0
Reviewed-on: https://chromium-review.googlesource.com/1068214
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563109}