Promote the downloads api to stable for M31!

BUG=246208
R=asargent
TBR=asargent

Review URL: https://codereview.chromium.org/23536067

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224531 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224531