Adding support for RTCRtpEncodingParameters.active flag.

This will allow a sender to stop/start sending media on the
application's demand.

Among other things, this can allow an application to set a track on a
sender while the encoding(s) are inactive, allowing the encoder to be
initialized for that track, then later set the encodings to "active"
to instantly start sending the track.

Review URL: https://codereview.webrtc.org/1822923002

Cr-Commit-Position: refs/heads/master@{#12094}
diff --git a/webrtc/api/rtpparameters.h b/webrtc/api/rtpparameters.h
index e71a1ae..2c29d98 100644
--- a/webrtc/api/rtpparameters.h
+++ b/webrtc/api/rtpparameters.h
@@ -18,6 +18,7 @@
 // These structures are defined as part of the RtpSender interface.
 // See http://w3c.github.io/webrtc-pc/#rtcrtpsender-interface for details.
 struct RtpEncodingParameters {
+  bool active = true;
   int max_bitrate_bps = -1;
 };