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

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

Index: chrome/browser/ui/webui/settings/md_settings_ui.cc
diff --git a/chrome/browser/ui/webui/settings/md_settings_ui.cc b/chrome/browser/ui/webui/settings/md_settings_ui.cc
index b2ddfdec737bb9f02687f965084f2af28bc433ea..913e7b7aaea780f2c1a209cbe33c94cffe051e54 100644
--- a/chrome/browser/ui/webui/settings/md_settings_ui.cc
+++ b/chrome/browser/ui/webui/settings/md_settings_ui.cc
@@ -8,6 +8,7 @@
 
 #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"
@@ -24,6 +25,7 @@
 #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"
@@ -80,8 +82,11 @@ MdSettingsUI::MdSettingsUI(content::WebUI* web_ui)
   AddSettingsPageUIHandler(new SystemHandler());
 #endif
 
+  std::string html_host = chrome::kChromeUIMdSettingsHost;

[groby-ooo-7-16, 2016/06/01 20:46:30]

Ugly, but necessary - since we use relative URLs, we'll need to root this at
chrome://settings if we serve from chrome://settings

So, the question is - do we still want to have chrome://md-settings around when
we enable MD from chrome://settings? The CL currently is halfway between. My
inclination is to say "no, let's just serve from one URL", but that's not quite
what we discussed on the previous CL. LMK

+  if (base::FeatureList::IsEnabled(features::kMaterialDesignSettingsFeature))
+    html_host = chrome::kChromeUISettingsHost;
   content::WebUIDataSource* html_source =
-      content::WebUIDataSource::Create(chrome::kChromeUIMdSettingsHost);
+      content::WebUIDataSource::Create(html_host);

[Dan Beam, 2016/06/01 20:52:57]

instead of |html_host|, could this web web_ui->GetURL().host()?

[Dan Beam, 2016/06/01 21:01:09]

web_ui->GetWebContents()->GetVisibleUrl().host() **

[groby-ooo-7-16, 2016/06/01 23:29:35]

Hm. I wonder if that could exploit routing problems (since now *any* URL that
manages to reach the handler will give whatever URL is currently visible
access.) 

[groby-ooo-7-16, 2016/06/02 23:03:53]

Tentatively done. Still feeling queasy about it.

 
 #if defined(OS_CHROMEOS)
   chromeos::settings::EasyUnlockSettingsHandler* easy_unlock_handler =