Add lots of testonly annotations to the GN build.

ui/views/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")
 
 gypi_values = exec_script(
     "//build/gypi_to_gn.py",
     [ rebase_path("views.gyp") ],
@@ skipping 86 matching lines @@
       } else if (is_win) {
         sources += gypi_values.views_desktop_aura_win_sources
       } else if (use_ozone) {
         sources += gypi_values.views_desktop_aura_ozone_sources
       }
     }
   }
 }
 
 static_library("test_support") {
+  testonly = true
   sources = gypi_values.views_test_support_sources
 
   deps = [
     ":views",
     "//base",
     "//ipc:test_support",
     "//skia",
     "//testing/gtest",
     "//ui/aura",
     "//ui/base",
@@ skipping 126 matching lines @@
       "//testing/gtest",
       "//ui/aura",
       "//ui/compositor",
       "//ui/resources",
       "//ui/resources:ui_test_pak",
       "//ui/strings",
       "//ui/wm",
     ]
   }
 }