Add lots of testonly annotations to the GN build.

ui/views/controls/webview/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("webview") {
   sources = [
     "unhandled_keyboard_event_handler.cc",
     "unhandled_keyboard_event_handler.h",
     "unhandled_keyboard_event_handler_linux.cc",
     "unhandled_keyboard_event_handler_win.cc",
@@ skipping 22 matching lines @@
     "//ui/views",
     "//url",
   ]
 
   forward_dependent_configs_from = [
     "//ui/views",
   ]
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
     "../../test/webview_test_helper.cc",
     "../../test/webview_test_helper.h",
   ]
 
   deps = [
     ":webview",
     "//base",
     "//content",
     "//content/test:test_support",
     "//ipc:test_support",
     "//skia",
     "//testing/gtest",
     "//ui/aura",
     "//ui/base",
     "//ui/events",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/views",
     "//ui/views:test_support",
   ]
 }