Mark gtest and gmock as testonly in GN.

components/enhanced_bookmarks/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.
 
 source_set("enhanced_bookmarks") {
   sources = [
     "image_store.cc",
     "image_store.h",
     "image_store_util.cc",
     "image_store_util.h",
@@ skipping 14 matching lines @@
   ]
 
   if (is_ios) {
     sources -= [
       "image_store_util.cc",
     ]
   }
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
     "test_image_store.cc",
     "test_image_store.h",
   ]
 
   deps = [
     ":enhanced_bookmarks",
     "//skia",
     "//testing/gtest",
   ]
 }