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}

common_types.h

@@ -509,6 +509,7 @@ struct SimulcastStream {
   unsigned int targetBitrate;  // kilobits/sec.
   unsigned int minBitrate;     // kilobits/sec.
   unsigned int qpMax;          // minimum quality
+  bool active;                 // encoded and sent.
 };
 
 struct SpatialLayer {
@@ -540,6 +541,10 @@ class VideoCodec {
 
   uint32_t maxFramerate;
 
+  // This enables/disables VP9 and H264 encoding/sending by allocating
+  // 0 bitrate if inactive.
+  bool active;
+
   unsigned int qpMax;
   unsigned char numberOfSimulcastStreams;
   SimulcastStream simulcastStream[kMaxSimulcastStreams];