MediaStream API: Patch #2 of implementing navigator.getMediaDevices
This patch just adds basic mock support so that getMediaDevices can
be exercised in blink layout tests.

BUG=338511

Review URL: https://codereview.chromium.org/136123005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248956 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248956