Add the possibility to specify MediaStreamTrack::remote amd MediaStreamTrack::_readonly when creating a source.

This is to support MediaStreamTrack::remote and MediaStreamTrack::_readonly as specified in http://w3c.github.io/mediacapture-main/getusermedia.html#widl-MediaStreamTrack-remote.

Note that this does not updated the IDL file or add tests since I want to update the content parts before allowing JS to access these properties.

BUG= 418540

Review URL: https://codereview.chromium.org/569613002

git-svn-id: svn://svn.chromium.org/blink/trunk@184246 bbb929c8-8fbe-4397-9dbb-9b2b20218538