Add lots of testonly annotations to the GN build.

media/cast/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 component("cast") {
   deps = [
     ":sender",
     ":receiver",
   ]
 }
@@ skipping 151 matching lines @@
     "//media",
     "//media:shared_memory_support",
     "//third_party/opus",
 # TODO(hclam): libvpx support.
 #    "//third_party/libvpx",
     "//ui/gfx",
   ]
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
 # TODO(hclam): FFmpeg.
 #    "test/fake_media_source.cc",
 #    "test/fake_media_source.h",
     "test/fake_single_thread_task_runner.cc",
     "test/fake_single_thread_task_runner.h",
     "test/skewed_single_thread_task_runner.cc",
     "test/skewed_single_thread_task_runner.h",
     "test/skewed_tick_clock.cc",
     "test/skewed_tick_clock.h",
@@ skipping 94 matching lines @@
     ":sender",
     ":test_support",
     "//base/test:run_all_unittests",
     "//base/test:test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }
 
 executable("generate_barcode_video") {
+  testonly = true
   sources = [
     "test/utility/generate_barcode_video.cc",
   ]
 
   deps = [
     ":test_support",
     "//base",
     "//media",
   ]
 }
 
 executable("generate_timecode_audio") {
+  testonly = true
   sources = [
     "test/utility/generate_timecode_audio.cc",
   ]
 
   deps = [
     ":test_support",
     "//base",
     "//media",
   ]
 }
 
 executable("udp_proxy") {
+  testonly = true
   sources = [
     "test/utility/udp_proxy_main.cc",
   ]
 
   deps = [
     ":test_support",
     "//base",
     "//net",
   ]
 }