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

This is a reland of 18c4261339dc76b220e7c805e36b4ea6f3dd161d
Original change's description:
> 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}

TBR=sprang@webrtc.org,stefan@webrtc.org,deadbeef@webrtc.org

Bug: webrtc:8630
Change-Id: Ib3df6f9b7158bff362a7ec66fc57e368682c5846
Reviewed-on: https://webrtc-review.googlesource.com/40980
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Seth Hampson <shampson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21688}
diff --git a/test/encoder_settings.cc b/test/encoder_settings.cc
index 84d3916..205af7b 100644
--- a/test/encoder_settings.cc
+++ b/test/encoder_settings.cc
@@ -50,6 +50,7 @@
         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;
   }