Mark gtest and gmock as testonly in GN.

media/base/android/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/android/rules.gni")
 import("//build/config/arm.gni")
 import("//build/config/ui.gni")
 import("//media/media_options.gni")
 
@@ skipping 34 matching lines @@
     "webaudio_media_codec_info.h",
   ]
   configs += [ "//media:media_config" ]
   deps = [
     ":media_jni_headers",
     "//third_party/widevine/cdm:version_h",
   ]
 }
 
 source_set("unittests") {
+  testonly = true
   sources = [
     "media_codec_bridge_unittest.cc",
     "media_drm_bridge_unittest.cc",
     "media_source_player_unittest.cc",
   ]
   deps = [
     ":android",
     "//media/base:test_support",
     "//testing/gmock",
     "//testing/gtest",
@@ skipping 17 matching lines @@
   jni_package = "media"
 }
 
 generate_jni("video_capture_jni_headers") {
   sources = [
     "java/src/org/chromium/media/VideoCapture.java",
     "java/src/org/chromium/media/VideoCaptureFactory.java",
   ]
   jni_package = "media"
 }