Mark gtest and gmock as testonly in GN.

media/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.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/ui.gni")
 import("//build/config/linux/pkg_config.gni")
 import("//media/media_options.gni")
 
@@ skipping 601 matching lines @@
     deps += [ "//ui/gl" ]
   }
   if (media_use_ffmpeg) {
     deps += [
       "//third_party/ffmpeg",  # Direct dependency required to inherit config.
     ]
   }
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
     "filters/clockless_video_frame_scheduler.cc",
     "filters/clockless_video_frame_scheduler.h",
     "filters/mock_gpu_video_accelerator_factories.cc",
     "filters/mock_gpu_video_accelerator_factories.h",
     "filters/test_video_frame_scheduler.cc",
     "filters/test_video_frame_scheduler.h",
     "video/mock_video_decode_accelerator.cc",
     "video/mock_video_decode_accelerator.h",
   ]
@@ skipping 114 matching lines @@
   java_cpp_template("media_android_imageformat_list") {
     sources = [
       "base/android/java/src/org/chromium/media/ImageFormat.template",
     ]
     inputs = [
       "video/capture/android/imageformat_list.h"
     ]
     package_name = "org/chromium/media"
   }
 }