MediaRecorder: Don't look at MediaStreamTracks IsEnabled()

As explained in crbug.com/878255#c13, MR should not take
into account MediaStreamTrack::IsEnabled() to start recording
a given MediaStreamTrack: when they are disabled, they produce
either black VideoFrames or all zero Audio -- but that's a
legit content and should be recorded.  This CL fixes that and
adds a LayoutTest.

TBR=foolip@chromium.org

Bug: 878255
Change-Id: I4be380e838d3287acae7737efb7075e195ac65fd
Reviewed-on: https://chromium-review.googlesource.com/c/1393538
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619985}
2 files changed