Add PeerConnectionInterface::UpdateCallBitrate.

webrtc/api/peerconnectioninterface.h

Index: webrtc/api/peerconnectioninterface.h
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index e96545736257bdbd25b27715bf48c828926d4dcb..07770b24c097dfb1b6423d59f64a1fe379c292b7 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -728,6 +728,16 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
   // destroyed, RegisterUMAOberver(nullptr) should be called.
   virtual void RegisterUMAObserver(UMAObserver* observer) = 0;
 
+  // Parameters that are not set will defer to implementation defined values.

[Taylor Brandstetter, 2017/04/12 01:33:41]

"or values from x-google-[min/start/max]-bitrate, or b=AS"? Probably worth
mentioning, as long as it works that way.

[Zach Stein, 2017/04/13 00:26:36]

I was being intentionally vague to avoid this getting out of sync in the future.
I meant for implementation defined values to include all the things we do now
(static constants, SDP, and codec parameters), but I see that it would be easy
to interpret as only being constants. Can you think of a more concise way to say
"the implementation is free to choose whatever values it wants for parameters
that are not set here?"

[Taylor Brandstetter, 2017/04/13 22:16:13]

If you were being intentionally vague, "implementation defined" sounds good.

+  struct BitrateParameters {
+    rtc::Optional<int> min_bitrate_bps;
+    rtc::Optional<int> start_bitrate_bps;
+    rtc::Optional<int> max_bitrate_bps;

[Taylor Brandstetter, 2017/04/12 01:33:41]

I think it's worth describing the behavior of min/start/max at a high level
here. For example, how changes to "start" will restart bandwidth estimation (I
believe that's what we found).

[Zach Stein, 2017/04/13 00:26:36]

Done.

+  };
+
+  // Updates the Call level bitrate parameters.

[Taylor Brandstetter, 2017/04/12 01:33:41]

I wouldn't say "Call level", since someone just using the PeerConnection API
won't know what that means.

[Zach Stein, 2017/04/13 00:26:36]

Done.

+  virtual void SetCallBitrate(const BitrateParameters& bitrate) = 0;
+
   // Returns the current SignalingState.
   virtual SignalingState signaling_state() = 0;
   virtual IceConnectionState ice_connection_state() = 0;