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

chrome/browser/browser_about_handler.cc

 // Copyright (c) 2012 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/browser_about_handler.h"
 
 #include <string>
 
 #include "base/bind.h"
 #include "base/location.h"
@@ skipping 75 matching lines @@
     // Material design history is handled on the top-level chrome://history
     // host.
     if (MdHistoryUI::IsEnabled(Profile::FromBrowserContext(browser_context))) {
       host = chrome::kChromeUIHistoryHost;
       path = url->path();
     } else {
       host = chrome::kChromeUIUberHost;
       path = chrome::kChromeUIHistoryHost + url->path();
     }
 #endif
-  // Redirect chrome://settings
+  // Redirect chrome://settings, unless MD settings is enabled.
   } else if (host == chrome::kChromeUISettingsHost) {
     if (::switches::AboutInSettingsEnabled()) {
       host = chrome::kChromeUISettingsFrameHost;
+    } else if (base::FeatureList::IsEnabled(
+                   features::kMaterialDesignSettingsFeature)) {
+      return true;  // Prevent further rewriting - this is a valid URL.
     } else {
       host = chrome::kChromeUIUberHost;
       path = chrome::kChromeUISettingsHost + url->path();
     }
-  // Redirect chrome://help
+  // Redirect chrome://help, unless MD settings is enabled.
   } else if (host == chrome::kChromeUIHelpHost) {
     if (::switches::AboutInSettingsEnabled()) {
       host = chrome::kChromeUISettingsFrameHost;
       if (url->path().empty() || url->path() == "/")
         path = chrome::kChromeUIHelpHost;
+    } else if (base::FeatureList::IsEnabled(
+                   features::kMaterialDesignSettingsFeature)) {
+      return true;  // Prevent further rewriting - this is a valid URL.
     } else {
       host = chrome::kChromeUIUberHost;
       path = chrome::kChromeUIHelpHost + url->path();
     }
   }
 
   GURL::Replacements replacements;
   replacements.SetHostStr(host);
   if (!path.empty())
     replacements.SetPathStr(path);
@@ skipping 13 matching lines @@
         FROM_HERE, base::Bind(&chrome::AttemptRestart));
     return true;
   } else if (base::LowerCaseEqualsASCII(spec, chrome::kChromeUIQuitURL)) {
     base::ThreadTaskRunnerHandle::Get()->PostTask(
         FROM_HERE, base::Bind(&chrome::AttemptExit));
     return true;
   }
 
   return false;
 }