MediaStream API: Updating NavigatorUserMediaError
Updating NavigatorUserMediaError both to reflect the latest spec but also
to make it a bit cleaner.

Review URL: https://chromiumcodereview.appspot.com/23444052

git-svn-id: svn://svn.chromium.org/blink/trunk@157697 bbb929c8-8fbe-4397-9dbb-9b2b20218538