PPAPI: Promote PPB_FileRef (v1.2) to stable

BUG=314879
TEST=browser_tests
R=dmichael@chromium.org

Review URL: https://codereview.chromium.org/135393005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249286 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249286