Pepper: specify the security origin when enumerating media devices.

BUG=312841
TEST=None
R=avi@chromium.org, bbudge@chromium.org, joi@chromium.org, wjia@chromium.org

Review URL: https://codereview.chromium.org/48743008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232926 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232926