Reland "Add first part of the network_tester functionality".

webrtc/tools/network_tester/network_tester_config.proto

Index: webrtc/tools/network_tester/network_tester_config.proto
diff --git a/webrtc/tools/network_tester/network_tester_config.proto b/webrtc/tools/network_tester/network_tester_config.proto
new file mode 100644
index 0000000000000000000000000000000000000000..54a23a2d83076e57ba5f7e1c49db062ca063e2ad
--- /dev/null
+++ b/webrtc/tools/network_tester/network_tester_config.proto
@@ -0,0 +1,13 @@
+syntax = "proto2";
+option optimize_for = LITE_RUNTIME;
+package webrtc.network_tester.config;
+
+message NetworkTesterConfig {
+  optional int32 packet_send_interval_ms = 1;
+  optional float packet_size = 2;
+  optional int32 execution_time_ms = 3;
+}
+
+message NetworkTesterAllConfigs {
+  repeated NetworkTesterConfig configs = 1;
+}