content_shell: avoid debug crashing for mid-clicks on links.

content_shell: avoid debug crashing for mid-clicks on links.

There was a DCHECK in content_shell that we only opened links in the
current tab. That made middle clicks crash a debug content_shell and
I need to test middle clicks in Blink so that is no good.

BUG=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=220281

[Daniel Bratell, 2013-08-27 18:06:32]

Just replacing a DCHECK with return NULL.

[esprehn, 2013-08-27 18:08:40]

Looks fine to me, but I don't have power out in content/ land. Adding jamesr@

[jamesr, 2013-08-28 22:17:37]

I don't either for here.  jochen@/pfeldman@ are the OWNERS for content/shell:
http://src.chromium.org/viewvc/chrome/trunk/src/content/shell/OWNERS, you want
them or a content/ owner.

[pfeldman, 2013-08-29 07:29:24]

lgtm

[commit-bot: I haz the power, 2013-08-29 07:35:47]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/bratell@opera.com/23517003/1

[commit-bot: I haz the power, 2013-08-29 07:51:36]

Step "update" is always a major failure.
Look at the try server FAQ for more details.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=android_cl...

[commit-bot: I haz the power, 2013-08-29 09:28:31]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/bratell@opera.com/23517003/1

[commit-bot: I haz the power, 2013-08-29 12:23:49]

Change committed as 220281