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}
diff --git a/webrtc/modules/audio_coding/BUILD.gn b/webrtc/modules/audio_coding/BUILD.gn
index 99fa140..3797818 100644
--- a/webrtc/modules/audio_coding/BUILD.gn
+++ b/webrtc/modules/audio_coding/BUILD.gn
@@ -791,6 +791,8 @@
     "neteq/neteq_impl.h",
     "neteq/normal.cc",
     "neteq/normal.h",
+    "neteq/packet.cc",
+    "neteq/packet.h",
     "neteq/packet_buffer.cc",
     "neteq/packet_buffer.h",
     "neteq/payload_splitter.cc",