DCHECK() that the source WebContents being passed to Browser::OpenURLInTab() is part of the tab strip.

R=sky@chromium.org
BUG=none

Depends on: https://codereview.chromium.org/114503003

Review URL: https://codereview.chromium.org/113113003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241256 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241256