Add flag enabling more packets to be retransmittable.

webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h

Index: webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
diff --git a/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h b/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
index 7d54ea4b24606b1f9692739924b0f6e71eda7da1..8add0a680a85b5bde7ea6d87877b2bd929d57812 100644
--- a/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
+++ b/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
@@ -113,11 +113,16 @@ enum KeyFrameRequestMethod { kKeyFrameReqPliRtcp, kKeyFrameReqFirRtcp };
 
 enum RtpRtcpPacketType { kPacketRtp = 0, kPacketKeepAlive = 1 };
 
+// kConditionallyRetransmitHigherLayers allows retransmission of video frames
+// in higher layers if either the last frame in that layer was too far back in
+// time, or if we estimate that a new frame will be available in a lower layer
+// in a shorter time than it would take to request and receive a retransmission.
 enum RetransmissionMode : uint8_t {
   kRetransmitOff = 0x0,
   kRetransmitFECPackets = 0x1,
   kRetransmitBaseLayer = 0x2,
   kRetransmitHigherLayers = 0x4,
+  kConditionallyRetransmitHigherLayers = 0x8,
   kRetransmitAllPackets = 0xFF
 };