Don't reject getUserMedia() requests with system audio when the feature is not supported.

Previously getUserMedia() requests for screencast with audio were
rejected when audio is not supported, even when video can still be
captured. Now such requests will succeed in, but returned
MediaStream will not have an audio track. The new behavior is more
consistent with how getUserMedia() works for webcam/microphone
streams (it always succeed, even when there is no microphone
available).

BUG=223639
R=tommi@chromium.org

Review URL: https://codereview.chromium.org/24153018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224871 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224871