[rAC, OSX] enable rAC by default for Dev/Canary

BUG=none

Review URL: https://codereview.chromium.org/24478003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225431 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225431