Prevent crash during sign in if user opens a second tab while still signing in
on a first tab.  Note that on windows, the enterprise dialog is app modal,
whereas on the other platforms its not.  However this fix is good for all
plaforms.  Essentially, check weak pointer before using it.

BUG=230295

Review URL: https://codereview.chromium.org/34013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230576 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230576