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Some links open a new tab while navigating, given that we redirect some
of the traffic to ARC, it can be necessary to close any unused tab.

Since we are closing the contained WebContents on a tab, we need to make
sure the callback within the UI doesn't get called twice, since intent
picker has an observer for the WebContents.

BUG=647806
TEST=manual test

Review-Url: https://codereview.chromium.org/2347273002
Cr-Commit-Position: refs/heads/master@{#419335}