Add AshDesktopMediaList and enable Desktop Capture API on Chrome OS.

The new AshDesktopMediaList gets list of windows/screens for the window
picker dialog which allows to enable Desktop Capture API on Chrome OS.

BUG=295102
R=ben@chromium.org, hshi@chromium.org

Review URL: https://codereview.chromium.org/100833002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242361 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242361