Fix window.open crashes on custom tabs

Fix window.open crashes on custom tabs

Overrides the tabwebcontentdelegate calls that
assumes the presence of a tabcreator which is not there for
custom tabs. Loads the targetUrl on the same page instead.

BUG=524562
Committed: https://crrev.com/7534de1539af204312dbfafa8fa8ab966a1e1989
Cr-Commit-Position: refs/heads/master@{#345525}

[Yusuf, 2015-08-26 00:32:07]

yusufo@chromium.org changed reviewers:
+ dfalcantara@chromium.org, mariakhomenko@chromium.org

[Maria, 2015-08-26 00:49:56]

lgtm

[Yusuf, 2015-08-26 00:53:04]

The CQ bit was checked by yusufo@chromium.org

[commit-bot: I haz the power, 2015-08-26 00:54:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1308563006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1308563006/1

[commit-bot: I haz the power, 2015-08-26 01:39:11]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-26 01:39:46]

Patchset 1 (id:??) landed as
https://crrev.com/7534de1539af204312dbfafa8fa8ab966a1e1989
Cr-Commit-Position: refs/heads/master@{#345525}