Enables/disables simulcast streams by allocating a bitrate of 0 to the spatial layer.

Creates VideoStreams & VideoCodec.simulcastStreams with an active field, and then allocates 0 bitrate to simulcast streams that are inactive. This turns off the encoder for specific simulcast streams.

Bug: webrtc:8653
Change-Id: Id93b03dcd8d1191a7d3300bd77882c8af96ee469
Reviewed-on: https://webrtc-review.googlesource.com/37740
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Seth Hampson <shampson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21646}

test/encoder_settings.cc

@@ -50,6 +50,7 @@ std::vector<VideoStream> CreateVideoStreams(
         std::min(bitrate_left_bps,
                  DefaultVideoStreamFactory::kMaxBitratePerStream[i]);
     stream_settings[i].max_qp = 56;
+    stream_settings[i].active = true;
     bitrate_left_bps -= stream_settings[i].target_bitrate_bps;
   }