Add lots of testonly annotations to the GN build.

ui/message_center/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/features.gni")
 import("//build/config/ui.gni")
 
 component("message_center") {
   defines = [ "MESSAGE_CENTER_IMPLEMENTATION" ]
 
@@ skipping 129 matching lines @@
         "notification.cc",
         "notification.h",
         "notifier_settings.cc",
         "notifier_settings.h",
       ]
     }
   }
 }
 
 static_library("test_support") {
+  testonly = true
   sources = [
     "fake_message_center.h",
     "fake_message_center.cc",
     "fake_message_center_tray_delegate.h",
     "fake_message_center_tray_delegate.cc",
     "fake_notifier_settings_provider.h",
     "fake_notifier_settings_provider.cc",
   ]
 
   deps = [
@@ skipping 59 matching lines @@
       deps += [
         # Compositor is needed by message_center_view_unittest.cc and for the
         # fonts used by bounded_label_unittest.cc.
         "//ui/compositor",
         "//ui/views",
         "//ui/views:test_support",
       ]
     }
   }  # enable_notifications
 }