roll webrtc 4819 -> 4831

R=ronghuawu@chromium.org

Review URL: https://codereview.chromium.org/24438002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225093 0039d316-1c4b-4281-b951-d872f2087c98
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225094 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225093
SVN changes up to revision 225094