[WebUI] Add a chrome://prefs-internals page

chrome/browser/ui/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/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
@@ skipping 373 matching lines @@
     "webui/policy_material_design_ui.cc",
     "webui/policy_material_design_ui.h",
     "webui/policy_ui.cc",
     "webui/policy_ui.h",
     "webui/policy_ui_handler.cc",
     "webui/policy_ui_handler.h",
     "webui/predictors/predictors_handler.cc",
     "webui/predictors/predictors_handler.h",
     "webui/predictors/predictors_ui.cc",
     "webui/predictors/predictors_ui.h",
+    "webui/prefs_internals_source.cc",
+    "webui/prefs_internals_source.h",
     "webui/profiler_ui.cc",
     "webui/profiler_ui.h",
     "webui/quota_internals/quota_internals_handler.cc",
     "webui/quota_internals/quota_internals_handler.h",
     "webui/quota_internals/quota_internals_proxy.cc",
     "webui/quota_internals/quota_internals_proxy.h",
     "webui/quota_internals/quota_internals_types.cc",
     "webui/quota_internals/quota_internals_types.h",
     "webui/quota_internals/quota_internals_ui.cc",
     "webui/quota_internals/quota_internals_ui.h",
@@ skipping 3124 matching lines @@
       "test/test_confirm_bubble_model.cc",
       "test/test_confirm_bubble_model.h",
     ]
     deps += [ "//chrome/test:test_support_ui" ]
   }
 
   if (enable_extensions) {
     deps += [ "//extensions/browser" ]
   }
 }