MediaStreamTrackSourcesRequest::create should take a const String&, not a String

As requested in comments on https://codereview.chromium.org/84923004/

BUG=297477

Review URL: https://codereview.chromium.org/113783004

git-svn-id: svn://svn.chromium.org/blink/trunk@163875 bbb929c8-8fbe-4397-9dbb-9b2b20218538