[MD Settings] Add feature to enable md-settings by default.

chrome/browser/ui/webui/settings/md_settings_ui.cc

 // Copyright 2015 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.
 
 #include "chrome/browser/ui/webui/settings/md_settings_ui.h"
 
 #include <stddef.h>
 
 #include <string>
 
+#include "base/feature_list.h"
 #include "base/metrics/histogram_macros.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/webui/settings/about_handler.h"
 #include "chrome/browser/ui/webui/settings/appearance_handler.h"
 #include "chrome/browser/ui/webui/settings/downloads_handler.h"
 #include "chrome/browser/ui/webui/settings/font_handler.h"
 #include "chrome/browser/ui/webui/settings/languages_handler.h"
 #include "chrome/browser/ui/webui/settings/md_settings_localized_strings_provider.h"
 #include "chrome/browser/ui/webui/settings/people_handler.h"
 #include "chrome/browser/ui/webui/settings/profile_info_handler.h"
 #include "chrome/browser/ui/webui/settings/reset_settings_handler.h"
 #include "chrome/browser/ui/webui/settings/search_engines_handler.h"
 #include "chrome/browser/ui/webui/settings/settings_clear_browsing_data_handler.h"
 #include "chrome/browser/ui/webui/settings/settings_page_ui_handler.h"
 #include "chrome/browser/ui/webui/settings/settings_startup_pages_handler.h"
 #include "chrome/browser/ui/webui/settings/site_settings_handler.h"
+#include "chrome/common/chrome_features.h"
 #include "chrome/common/url_constants.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/browser/web_ui.h"
 #include "content/public/browser/web_ui_data_source.h"
 #include "grit/settings_resources.h"
 #include "grit/settings_resources_map.h"
 
 #if defined(OS_CHROMEOS)
 #include "chrome/browser/ui/webui/settings/chromeos/change_picture_handler.h"
 #include "chrome/browser/ui/webui/settings/chromeos/device_keyboard_handler.h"
@@ skipping 36 matching lines @@
 
 #if defined(OS_CHROMEOS)
   AddSettingsPageUIHandler(new chromeos::settings::ChangePictureHandler());
   AddSettingsPageUIHandler(new chromeos::settings::KeyboardHandler(web_ui));
 #else
   AddSettingsPageUIHandler(new DefaultBrowserHandler(web_ui));
   AddSettingsPageUIHandler(new ManageProfileHandler(profile));
   AddSettingsPageUIHandler(new SystemHandler());
 #endif
 
-  content::WebUIDataSource* html_source =
-      content::WebUIDataSource::Create(chrome::kChromeUIMdSettingsHost);
+  // Host must be derived from the visible URL, since this might be serving
+  // either chrome://settings or chrome://md-settings.

[Dan Beam, 2016/06/03 03:01:43]

if you're worried about it,

const GURL url = web_ui->GetWebContents()->GetVisibleURL();
DCHECK(url.GetOrigin() == GURL(chrome::kChromeUISettingsHost).GetOrigin() ||
       url.GetOrigin() == GURL(chrome::kChromeUIMdSettingsHost).GetOrigin());
content::WebUIDataSource* html_source =
    content::WebUIDataSource::Create(url.host());

[groby-ooo-7-16, 2016/06/06 20:11:59]

Done. And given that this is a possible privilege escalation, it's a CHECK. 

+  content::WebUIDataSource* html_source = content::WebUIDataSource::Create(
+      web_ui->GetWebContents()->GetVisibleURL().host());
 
 #if defined(OS_CHROMEOS)
   chromeos::settings::EasyUnlockSettingsHandler* easy_unlock_handler =
       chromeos::settings::EasyUnlockSettingsHandler::Create(html_source,
                                                             profile);
   if (easy_unlock_handler)
     AddSettingsPageUIHandler(easy_unlock_handler);
 #endif
 
   AddSettingsPageUIHandler(AboutHandler::Create(html_source, profile));
@@ skipping 34 matching lines @@
   UMA_HISTOGRAM_TIMES("Settings.LoadDocumentTime.MD",
                       base::Time::Now() - load_start_time_);
 }
 
 void MdSettingsUI::DocumentOnLoadCompletedInMainFrame() {
   UMA_HISTOGRAM_TIMES("Settings.LoadCompletedTime.MD",
                       base::Time::Now() - load_start_time_);
 }
 
 }  // namespace settings