Add an accessibility messages specifically for location changes.

Location changes are special-cased because they're frequent and it's
inefficient recompute any other accessibility properties. However, it
was a hack to send them as part of a regular accessibility event IPC -
putting them in their own IPC is cleaner.

BUG=none
R=dtseng@chromium.org, jam@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/116853005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242890 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242890