Mark gtest and gmock as testonly in GN.

components/gcm_driver/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.
 
 static_library("gcm_driver") {
   sources = [
     "android/component_jni_registrar.cc",
     "android/component_jni_registrar.h",
     "default_gcm_app_handler.cc",
     "default_gcm_app_handler.h",
@@ skipping 48 matching lines @@
     ]
     deps -= [
       "//components/gcm_driver/proto",
       "//google_apis/gcm",
     ]
     deps += [ ":jni_headers" ]
   }
 }
 
 static_library("test_support") {
+  testonly = true
   sources = [
     "fake_gcm_app_handler.cc",
     "fake_gcm_app_handler.h",
     "fake_gcm_client.cc",
     "fake_gcm_client.h",
     "fake_gcm_client_factory.cc",
     "fake_gcm_client_factory.h",
     "fake_gcm_driver.cc",
     "fake_gcm_driver.h",
   ]
@@ skipping 10 matching lines @@
       "fake_gcm_client.cc",
       "fake_gcm_client.h",
       "fake_gcm_client_factory.cc",
       "fake_gcm_client_factory.h",
     ]
     deps -= [ "//google_apis/gcm" ]
   }
 }
 
 #TODO(GYP) gcm_driver_java gcm_driver_jni_headers