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This crash would happen if you select a couple of tabs, then click but
don't drag and hit escape or control-w. The crash would occur because
we would end up inserting the tabs a second time even though they are
already in the tabstrip.

BUG=360822
TEST=see bug
R=ben@chromium.org

Review URL: https://codereview.chromium.org/548703002

Cr-Commit-Position: refs/heads/master@{#293625}