Add support for enabling simulcast in "Plan B" using MediaConstraints.

BUG=webrtc:9655

Change-Id: Ieb5fe5d97b6d4381608a51593bca5423979d1b9f
Reviewed-on: https://webrtc-review.googlesource.com/95481
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24424}

api/mediaconstraintsinterface.h

@@ -119,6 +119,11 @@ class MediaConstraintsInterface {
   // stripped by Chrome before passed down to Libjingle.
   static const char kInternalConstraintPrefix[];
 
+  // Specifies number of simulcast layers for all video tracks
+  // with a Plan B offer/answer
+  // (see RTCOfferAnswerOptions::num_simulcast_layers).
+  static const char kNumSimulcastLayers[];
+
   virtual ~MediaConstraintsInterface() = default;
 
   virtual const Constraints& GetMandatory() const = 0;