audio/red: provide default fmtp line

otherwise the generated codec won't match the preassigned codec
and red will use 96 as payload type, increasing the payload type
congestion in the upper range.

BUG=webrtc:11640

Change-Id: I466ed6d4e025ef116f3099e85855e10493408ab1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/233560
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35130}

media/engine/payload_type_mapper.cc

@@ -73,7 +73,12 @@ PayloadTypeMapper::PayloadTypeMapper()
               {kCodecParamUseInbandFec, kParamValueTrue}}},
             111},
            // RED for opus is assigned in the lower range, starting at the top.
-           {{kRedCodecName, 48000, 2}, 63},
+           // Note that the FMTP refers to the opus payload type.
+           {{kRedCodecName,
+             48000,
+             2,
+             {{kCodecParamNotInNameValueFormat, "111/111"}}},
+            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

@@ -58,7 +58,8 @@ TEST_F(PayloadTypeMapperTest, WebRTCPayloadTypes) {
                       48000,
                       2,
                       {{"minptime", "10"}, {"useinbandfec", "1"}}}));
-  EXPECT_EQ(63, mapper_.FindMappingFor({kRedCodecName, 48000, 2}));
+  EXPECT_EQ(
+      63, mapper_.FindMappingFor({kRedCodecName, 48000, 2, {{"", "111/111"}}}));
   // TODO(solenberg): Remove 16k, 32k, 48k DTMF checks once these payload types
   // are dynamically assigned.
   EXPECT_EQ(110, mapper_.FindMappingFor({kDtmfCodecName, 48000, 1}));