Set up Sphinx's linkchecker and fix some remaining bad links

BUG=None
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/27252003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228555 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228555