Fix Chromium clang plugin warnings

This adds a new file, webrtc/modules/audio_coding/neteq/tools/packet_source.cc, so that I'll have somewhere to put the new non-inlined methods.

NOTRY=true
BUG=webrtc:163

Review-Url: https://codereview.webrtc.org/2290593002
Cr-Commit-Position: refs/heads/master@{#13956}

webrtc/modules/audio_coding/neteq/tools/packet.cc

@@ -77,6 +77,8 @@ Packet::Packet(uint8_t* packet_memory,
   valid_header_ = ParseHeader(*parser);
 }
 
+Packet::~Packet() = default;
+
 bool Packet::ExtractRedHeaders(std::list<RTPHeader*>* headers) const {
   //
   //  0                   1                    2                   3