Support removing b=AS bandwidth constraints.

webrtc/media/engine/webrtcvideoengine2.cc

Index: webrtc/media/engine/webrtcvideoengine2.cc
diff --git a/webrtc/media/engine/webrtcvideoengine2.cc b/webrtc/media/engine/webrtcvideoengine2.cc
index 6723b1b89fd466fa4b5332eabfd5c3f20215612a..89ea5ced54eefc270dd2d1ef304a52dfecce7200 100644
--- a/webrtc/media/engine/webrtcvideoengine2.cc
+++ b/webrtc/media/engine/webrtcvideoengine2.cc
@@ -715,8 +715,10 @@ bool WebRtcVideoChannel2::GetChangedSendParameters(
 
   // Handle max bitrate.
   if (params.max_bandwidth_bps != send_params_.max_bandwidth_bps &&
-      params.max_bandwidth_bps >= 0) {
-    // 0 uncaps max bitrate (-1).
+      params.max_bandwidth_bps >= -1) {
+    // 0 or -1 uncaps max bitrate.
+    // TODO(pbos): Reconsider whether 0 should be invalid or not. -1 is the
+    // auto-bitrate value.

[Taylor Brandstetter, 2017/03/09 23:48:38]

The "correct" behavior for b=AS:0 may be "set bitrate to 0, don't send any
bits". I can't find any reference to 0 being a special value. But we should
change that separately.

[pbos-webrtc, 2017/03/10 18:52:09]

TODO text updated.

     changed_params->max_bandwidth_bps = rtc::Optional<int>(
         params.max_bandwidth_bps == 0 ? -1 : params.max_bandwidth_bps);
   }
@@ -762,7 +764,7 @@ bool WebRtcVideoChannel2::SetSendParameters(const VideoSendParameters& params) {
   if (changed_params.codec || changed_params.max_bandwidth_bps) {
     if (send_codec_) {
       // TODO(holmer): Changing the codec parameters shouldn't necessarily mean
-      // that we change the min/max of bandwidth estimation. Reevaluate this.
+      // that we change the min of bandwidth estimation. Reevaluate this.
       bitrate_config_ = GetBitrateConfigForCodec(send_codec_->codec);
       if (!changed_params.codec) {
         // If the codec isn't changing, set the start bitrate to -1 which means
@@ -770,7 +772,7 @@ bool WebRtcVideoChannel2::SetSendParameters(const VideoSendParameters& params) {
         bitrate_config_.start_bitrate_bps = -1;
       }
     }
-    if (params.max_bandwidth_bps >= 0) {
+    if (params.max_bandwidth_bps >= -1) {
       // Note that max_bandwidth_bps intentionally takes priority over the
       // bitrate config for the codec. This allows FEC to be applied above the
       // codec target bitrate.
@@ -778,6 +780,9 @@ bool WebRtcVideoChannel2::SetSendParameters(const VideoSendParameters& params) {
       // WebRtcVideoChannel2 (in which case we're good), or per sender (SSRC),
       // in which case this should not set a Call::BitrateConfig but rather
       // reconfigure all senders.
+
+      // TODO(pbos): Consider whether 0 should be invalid or not.
+      // 0 or -1 uncaps max bitrate.
       bitrate_config_.max_bitrate_bps =
           params.max_bandwidth_bps == 0 ? -1 : params.max_bandwidth_bps;
     }