Revert "Extensions: tab.title should not be undefined."

On vista, this change hard breaks tab.title in extensions.

This reverts commit cabda9d732b49ed018821772398d732bbc1af75a (r26586).

TBR=rafaelw

Review URL: http://codereview.chromium.org/207032

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26592 0039d316-1c4b-4281-b951-d872f2087c98