Move most of the remaining users of WebContentsObserver::tab_contents() to use web_contents().

BUG=98716
TBR=joi
Review URL: http://codereview.chromium.org/9030010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@115800 0039d316-1c4b-4281-b951-d872f2087c98
51 files changed