Adding C++ versions of currently spec'd "RtpParameters" structs.

These structs will be used for ORTC objects (and their WebRTC
equivalents).

This CL also introduces some minor changes to the existing implemented
structs:

- max_bitrate_bps uses rtc::Optional instead of "-1 means unset"
- "mime_type" turned into "name"/"kind" (which can be used to form the
  MIME type string, if needed).
- clock_rate and channels changed to rtc::Optional, since they will
  need to be for RtpSender.send().
- Renamed "channels" to "num_channels" (the ORTC name, which I prefer).

BUG=webrtc:7013, webrtc:7112

Review-Url: https://codereview.webrtc.org/2651883010
Cr-Commit-Position: refs/heads/master@{#16437}

webrtc/sdk/android/api/org/webrtc/RtpParameters.java

@@ -15,23 +15,43 @@ import java.util.LinkedList;
 /**
  * The parameters for an {@code RtpSender}, as defined in
  * http://w3c.github.io/webrtc-pc/#rtcrtpsender-interface.
+ *
+ * Note: These structures use nullable Integer/etc. types because in the
+ * future, they may be used to construct ORTC RtpSender/RtpReceivers, in
+ * which case "null" will be used to represent "choose the implementation
+ * default value".
  */
 public class RtpParameters {
   public static class Encoding {
+    // Set to true to cause this encoding to be sent, and false for it not to
+    // be sent.
     public boolean active = true;
-    // A null value means "no maximum bitrate".
+    // If non-null, this represents the Transport Independent Application
+    // Specific maximum bandwidth defined in RFC3890. If null, there is no
+    // maximum bitrate.
     public Integer maxBitrateBps;
+    // SSRC to be used by this encoding.
+    // Can't be changed between getParameters/setParameters.
     public Long ssrc;
   }
 
   public static class Codec {
-    int payloadType;
-    String mimeType;
-    int clockRate;
-    int channels = 1;
+    // Payload type used to identify this codec in RTP packets.
+    public int payloadType;
+    // Name used to identify the codec. Equivalent to MIME subtype.
+    public String name;
+    // The media type of this codec. Equivalent to MIME top-level type.
+    MediaStreamTrack.MediaType kind;
+    // Clock rate in Hertz.
+    public Integer clockRate;
+    // The number of audio channels used. Set to null for video codecs.
+    public Integer numChannels;
   }
 
   public final LinkedList<Encoding> encodings;
+  // Codec parameters can't currently be changed between getParameters and
+  // setParameters. Though in the future it will be possible to reorder them or
+  // remove them.
   public final LinkedList<Codec> codecs;
 
   public RtpParameters() {