Add lots of testonly annotations to the GN build.

ui/wm/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.
 
 component("wm") {
   sources = [
     "core/accelerator_delegate.h",
     "core/accelerator_filter.cc",
     "core/accelerator_filter.h",
     "core/base_focus_rules.cc",
@@ skipping 72 matching lines @@
     "//ui/events/platform",
     "//ui/events:events_base",
     "//ui/events/platform",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/resources",
   ]
 }
 
 static_library("test_support") {
+  testonly = true
   sources = [
     "test/wm_test_helper.cc",
     "test/wm_test_helper.h",
   ]
 
   deps = [
     ":wm",
     "//skia",
     "//ui/aura",
     "//ui/aura:test_support",
@@ skipping 32 matching lines @@
     "//ui/base",
     "//ui/compositor",
     "//ui/events",
     "//ui/events:events_base",
     "//ui/events/platform",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/gl",
   ]
 }