Removed RTPHeader from NetEq's Packet struct.

Only three items in the (rather large) header were actually used after
InsertPacket: payloadType, timestamp and sequenceNumber. They are now
put directly into Packet. This saves 129 bytes per Packet that no
longer need to be allocated and deallocated.

This also works towards decoupling NetEq from RTP. As part of that,
I've moved the NACK code earlier in InsertPacketInternal, together
with other things that directly reference the RTPHeader.

BUG=webrtc:6549

Review-Url: https://codereview.webrtc.org/2411183003
Cr-Commit-Position: refs/heads/master@{#14658}

webrtc/modules/audio_coding/neteq/decoder_database_unittest.cc

@@ -176,7 +176,7 @@ TEST(DecoderDatabase, CheckPayloadTypes) {
     // Create packet with payload type |i|. The last packet will have a payload
     // type that is not registered in the decoder database.
     Packet* packet = new Packet;
-    packet->header.payloadType = i;
+    packet->payload_type = i;
     packet_list.push_back(packet);
   }