Check that TabContents is valid (not null) before removing theme change InfoBar.

      BUG=60761
TEST=interactive_ui_tests and repro steps in bug

Review URL: http://codereview.chromium.org/5326011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@68112 0039d316-1c4b-4281-b951-d872f2087c98