Add first part of the network_tester functionality.

webrtc/tools/network_tester/network_tester_packet.proto

+syntax = "proto2";
+option optimize_for = LITE_RUNTIME;
+package webrtc.network_tester.packet;
+
+message NetworkTesterPacket {
+  enum Type {
+    HI = 0;

[minyue-webrtc, 2017/04/03 08:05:19]

HI -> HAND_SHAKING / TEST_START?

[michaelt, 2017/04/03 13:05:39]

Done.

+    START_SEND = 1;
+    TEST_DATA = 2;
+    TEST_DONE = 3;
+  }
+
+  required Type type = 1;

[terelius, 2017/03/30 11:33:42]

While I don't have a strong opinion, please be aware that the "required" keyword
is *very* controversial. The basic problem is that required fields can't be
added or removed without breaking all old clients, which makes it hard to
maintain code once it has reached production. The standard way to avoid this is
to make all fields optional and write custom validation code.

The keywords "optional" and "required" keywords have been removed in proto3 for
this very reason.

[michaelt, 2017/04/03 13:05:39]

Ok will make this optional as well an check it in the code.

+  optional int64 send_timestamp = 2;
+  optional int64 arrival_timestamp = 3;
+  optional int64 sequence_number = 4;
+  optional int32 packet_size = 5;
+}