Add functionality to set min/max bitrate per simulcast layer through RtpEncodingParameters.

Target bitrate is set to 0.75 of the max bitrate.

Bug: webrtc:9341, webrtc:8655
Change-Id: I9a8c8bb95bb1532d45f05578832418464452340e
Reviewed-on: https://webrtc-review.googlesource.com/79821
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23676}

api/rtpsenderinterface.h

@@ -57,6 +57,7 @@ class RtpSenderInterface : public rtc::RefCountInterface {
   virtual RtpParameters GetParameters() = 0;
   // Note that only a subset of the parameters can currently be changed. See
   // rtpparameters.h
+  // The encodings are in increasing quality order for simulcast.
   virtual RTCError SetParameters(const RtpParameters& parameters) = 0;
 
   // Returns null for a video sender.