Change the default constraint behaviour for sourceID for gUm to optional.
Ie- currently Chrome can only handle one sourceID per guM request and in https://codereview.chromium.org/34393006 I decided to support it as a mandatory constraint. That was the wrong decision- especially since it turned out that sourceID specified as a mandatory constraint do not work at all and never has (crbug/325153).

TEST= https://src.chromium.org/chrome/trunk/src/chrome/test/data/webrtc/manual/peerconnection.html - modify a sourceID in the constraint thing to something invalid and click Request UserMedia. See that you get audio and video anyway.
BUG=269139
R=joi@chromium.org
TBR=tommi@chromium.org

Review URL: https://codereview.chromium.org/104603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238706 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238706