Fixes possible crash in tab dragging

Fixes possible crash in tab dragging

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

Committed: https://crrev.com/7a27d4d62877348f8e172c4a432cc54c0a969c02
Cr-Commit-Position: refs/heads/master@{#293625}

[sky, 2014-09-05 23:11:49]

Sure would be nice to have test coverage, but that proves painful for many
different reasons. I need to spend some quality time to make tests easier to
write for this code.

[Ben Goodger (Google), 2014-09-05 23:12:46]

lgtm

[sky, 2014-09-05 23:13:16]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2014-09-05 23:14:34]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sky@chromium.org/548703002/1

[commit-bot: I haz the power, 2014-09-06 01:28:00]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-06 01:28:00]

Try jobs failed on following builders:
  android_dbg_tests_recipe on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_dbg_tes...)

[sky, 2014-09-06 02:29:49]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2014-09-06 02:31:43]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sky@chromium.org/548703002/1

[commit-bot: I haz the power, 2014-09-06 11:18:52]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-06 11:18:53]

Exceeded time limit waiting for builds to trigger.

[sky, 2014-09-06 15:10:51]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2014-09-06 15:11:21]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sky@chromium.org/548703002/1

[commit-bot: I haz the power, 2014-09-06 18:56:02]

Committed patchset #1 (id:1) as 153da9bac982a0a7fd551b13372465b0a10562c5

[commit-bot: I haz the power, 2014-09-10 03:42:58]

Patchset 1 (id:??) landed as
https://crrev.com/7a27d4d62877348f8e172c4a432cc54c0a969c02
Cr-Commit-Position: refs/heads/master@{#293625}