Expose method to tell if a TabBase is initialized

BUG=306145
NOTRY=true

Review URL: https://codereview.chromium.org/108363006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239717 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239717