Fixed about://performance crash when the performance-monitor-gathering flag is disabled

R=yoz@chromium.org
BUG=

Review URL: https://codereview.chromium.org/26906002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228029 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228029