Settings Override API: extensions now can override the homepage.

BUG=267510
TBR=davemoore@chromium.org

Review URL: https://codereview.chromium.org/40693002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231001 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231001