NetEq: Use TickTimer in PacketBuffer

This change makes use of the TickTimer::Stopwatch in Packets. When a
packet is inserted into the PacketBuffer, a Stopwatch object is
attached to it. When the packet is extracted from the buffer, the
Stopwatch is read to know how long the packet waited in the buffer.

BUG=webrtc:5608

Review URL: https://codereview.webrtc.org/1917913002

Cr-Commit-Position: refs/heads/master@{#12508}

webrtc/modules/audio_coding/neteq/mock/mock_packet_buffer.h

@@ -19,8 +19,8 @@ namespace webrtc {
 
 class MockPacketBuffer : public PacketBuffer {
  public:
-  MockPacketBuffer(size_t max_number_of_packets)
-      : PacketBuffer(max_number_of_packets) {}
+  MockPacketBuffer(size_t max_number_of_packets, const TickTimer* tick_timer)
+      : PacketBuffer(max_number_of_packets, tick_timer) {}
   virtual ~MockPacketBuffer() { Die(); }
   MOCK_METHOD0(Die, void());
   MOCK_METHOD0(Flush,