Change type of pid_diff (int16_t -> uint8_t) according to updates in RTP payload profile.

webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc b/webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc
index 0e76a8eae82bf3a2246e16711f055b0ca59b81e7..6a00724096343d9cfe80aa55a97ee56dacfb5c28 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc
@@ -246,7 +246,7 @@ bool WriteRefIndices(const RTPVideoHeaderVP9& vp9,
       vp9.num_ref_pics == 0 || vp9.num_ref_pics > kMaxVp9RefPics) {
     return false;
   }
-  for (size_t i = 0; i < vp9.num_ref_pics; ++i) {
+  for (uint8_t i = 0; i < vp9.num_ref_pics; ++i) {
     bool n_bit = !(i == vp9.num_ref_pics - 1);
     RETURN_FALSE_ON_ERROR(writer->WriteBits(vp9.pid_diff[i], 7));
     RETURN_FALSE_ON_ERROR(writer->WriteBits(n_bit ? 1 : 0, 1));
@@ -301,7 +301,7 @@ bool WriteSsData(const RTPVideoHeaderVP9& vp9, rtc::BitBufferWriter* writer) {
         writer->WriteBits(vp9.gof.temporal_up_switch[i] ? 1 : 0, 1));
     RETURN_FALSE_ON_ERROR(writer->WriteBits(vp9.gof.num_ref_pics[i], 2));
     RETURN_FALSE_ON_ERROR(writer->WriteBits(kReservedBitValue0, 2));
-    for (size_t r = 0; r < vp9.gof.num_ref_pics[i]; ++r) {
+    for (uint8_t r = 0; r < vp9.gof.num_ref_pics[i]; ++r) {
       RETURN_FALSE_ON_ERROR(writer->WriteUInt8(vp9.gof.pid_diff[i][r]));
     }
   }
@@ -466,7 +466,7 @@ bool ParseSsData(rtc::BitBuffer* parser, RTPVideoHeaderVP9* vp9) {
     vp9->gof.temporal_up_switch[i] = u_bit ? true : false;
     vp9->gof.num_ref_pics[i] = r;
 
-    for (size_t p = 0; p < vp9->gof.num_ref_pics[i]; ++p) {
+    for (uint8_t p = 0; p < vp9->gof.num_ref_pics[i]; ++p) {
       uint8_t p_diff;
       RETURN_FALSE_ON_ERROR(parser->ReadUInt8(&p_diff));
       vp9->gof.pid_diff[i][p] = p_diff;