Mark gtest and gmock as testonly in GN.

components/sessions/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.
 
 if (is_android) {
   import("//build/config/android/config.gn")
 }
 
 component("sessions") {
   sources = [
@@ skipping 11 matching lines @@
     "//skia",
     "//url",
   ]
 
   if (!is_android || !is_android_webview_build) {
     deps += [ "//sync" ]
   }
 }
 
 static_library("test_support") {
+  testonly = true
   sources = [
     "serialized_navigation_entry_test_helper.cc",
     "serialized_navigation_entry_test_helper.h",
   ]
 
   deps = [
     "//skia",
     "//testing/gtest",
   ]
 
   if (!is_android || !is_android_webview_build) {
     deps += [ "//sync" ]
   }
 }