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/packet_buffer.h

@@ -14,6 +14,7 @@
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/base/optional.h"
 #include "webrtc/modules/audio_coding/neteq/packet.h"
+#include "webrtc/modules/include/module_common_types.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
@@ -80,9 +81,9 @@ class PacketBuffer {
   virtual int NextHigherTimestamp(uint32_t timestamp,
                                   uint32_t* next_timestamp) const;
 
-  // Returns a (constant) pointer the RTP header of the first packet in the
-  // buffer. Returns NULL if the buffer is empty.
-  virtual const RTPHeader* NextRtpHeader() const;
+  // Returns a (constant) pointer to the first packet in the buffer. Returns
+  // NULL if the buffer is empty.
+  virtual const Packet* PeekNextPacket() const;
 
   // Extracts the first packet in the buffer and returns a pointer to it.
   // Returns NULL if the buffer is empty. The caller is responsible for deleting