red: assign payload type 63 to audio/RED for opus

Starting new audio codecs from the top of the lower range
reduces collisions with video codecs which are assigned from
the bottom of the lower range

BUG=webrtc:11640

Change-Id: If6d2b849b8e1de777a1d4352df533e4f1845fde9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/220022
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34166}

media/engine/payload_type_mapper.cc

@@ -72,6 +72,8 @@ PayloadTypeMapper::PayloadTypeMapper()
              {{kCodecParamMinPTime, "10"},
               {kCodecParamUseInbandFec, kParamValueTrue}}},
             111},
+           // RED for opus is assigned in the lower range, starting at the top.
+           {{kRedCodecName, 48000, 2}, 63},
            // TODO(solenberg): Remove the hard coded 16k,32k,48k DTMF once we
            // assign payload types dynamically for send side as well.
            {{kDtmfCodecName, 48000, 1}, 110},

media/engine/payload_type_mapper_unittest.cc

@@ -58,6 +58,7 @@ TEST_F(PayloadTypeMapperTest, WebRTCPayloadTypes) {
                       48000,
                       2,
                       {{"minptime", "10"}, {"useinbandfec", "1"}}}));
+  EXPECT_EQ(63, mapper_.FindMappingFor({kRedCodecName, 48000, 2}));
   // TODO(solenberg): Remove 16k, 32k, 48k DTMF checks once these payload types
   // are dynamically assigned.
   EXPECT_EQ(110, mapper_.FindMappingFor({kDtmfCodecName, 48000, 1}));