Reland issue 27748003: Moves the ChromeVox extension into the Chrome source tree.

BUG=309130
TEST=run ChromeVox on Aura.
TBR=robliao

Review URL: https://codereview.chromium.org/100513003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239068 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239068