Revert of Revert of views: Move TOOLKIT_VIEWS definition in //ui/views/ from root.

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")
 import("//testing/test.gni")
 
 gypi_values = exec_script("//build/gypi_to_gn.py",
                           [ rebase_path("views.gyp") ],
                           "scope",
                           [ "views.gyp" ])
 
+config("flags") {
+  defines = [ "TOOLKIT_VIEWS=1" ]
+}
+
 component("views") {
+  all_dependent_configs = [ ":flags" ]
   sources = gypi_values.views_sources
 
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
   defines = [ "VIEWS_IMPLEMENTATION" ]
 
   deps = [
     "//base:i18n",
     "//base/third_party/dynamic_annotations",
@@ skipping 262 matching lines @@
       "//skia",
       "//testing/gtest",
       "//ui/base:test_support",
       "//ui/compositor",
       "//ui/resources",
       "//ui/resources:ui_test_pak",
       "//ui/strings",
     ]
   }
 }