MediaStream API: Addding support for UA specific error messages for gUM
The spec now says that getUserMedia error messages are UA specific with
the exception for PermissionDeniedError and ConstraintNotSatisfied.

BUG=132812

Review URL: https://codereview.chromium.org/180683003

git-svn-id: svn://svn.chromium.org/blink/trunk@168562 bbb929c8-8fbe-4397-9dbb-9b2b20218538