Remove a lot of dead code from the PrerenderTracker.

A lot of this is unused with the recent refactoring.

BUG=none
TEST=compiles

Review URL: https://codereview.chromium.org/143693008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247374 0039d316-1c4b-4281-b951-d872f2087c98
Add aboxhall to content/browser/accessibility/OWNERS

BUG=

Review URL: https://codereview.chromium.org/148403005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247375 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247375