GitLab

If the model association happens at the right time (i.e. when a tab is being
dragged from one window to another), the tab strip model may be out of date.
Just handle this gracefully for now and return false.

BUG=282590
TBR=haitao@chromium.org

Review URL: https://codereview.chromium.org/81193003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238338 0039d316-1c4b-4281-b951-d872f2087c98