Hide BrowsingInstance from all but SiteInstance, as intended by design.

chrome/browser/extensions/extension_process_manager.h

Index: chrome/browser/extensions/extension_process_manager.h
diff --git a/chrome/browser/extensions/extension_process_manager.h b/chrome/browser/extensions/extension_process_manager.h
index 484d36be382d33466c4ebe0baffc29a43dab285f..127086732251bbaa1a9b3e34284261a7b365b867 100644
--- a/chrome/browser/extensions/extension_process_manager.h
+++ b/chrome/browser/extensions/extension_process_manager.h
@@ -17,7 +17,6 @@
 #include "content/public/browser/notification_registrar.h"
 
 class Browser;
-class BrowsingInstance;
 class Extension;
 class ExtensionHost;
 class GURL;
@@ -118,9 +117,10 @@ class ExtensionProcessManager : public content::NotificationObserver {
   // The set of running viewless background extensions.
   ExtensionHostSet background_hosts_;
 
-  // The BrowsingInstance shared by all extensions in this profile.  This
-  // controls process grouping.
-  scoped_refptr<BrowsingInstance> browsing_instance_;
+  // A SiteInstance related to the SiteInstance for all extensions in
+  // this profile.  We create it in such a way that a new
+  // browsing instance is created.  This controls process grouping.
+  scoped_refptr<SiteInstance> site_instance_;
 
  private:
   // Contains all extension-related RenderViewHost instances for all extensions.