Mac: fix yet another TabStripController index/model index bug (crasher).
Mac: fix yet another TabStripController index/model index bug (crasher).

BUG=29906
TEST=Create a bunch of tabs; click on some close tab button while holding shift down to slow down animation; while animating, select another tab; repeat many, many times. Should not crash.

Review URL: http://codereview.chromium.org/553068

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@37010 0039d316-1c4b-4281-b951-d872f2087c98

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