Set the site instance in all cases when we start a new load. If you navigate ...

chrome/browser/tab_contents/render_view_host_manager.cc

 // Copyright (c) 2010 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/tab_contents/render_view_host_manager.h"
 
 #include "base/command_line.h"
 #include "base/logging.h"
 #include "chrome/browser/dom_ui/dom_ui.h"
 #include "chrome/browser/dom_ui/dom_ui_factory.h"
@@ skipping 321 matching lines @@
   //
   if (CommandLine::ForCurrentProcess()->HasSwitch(switches::kProcessPerSite) &&
       entry.transition_type() == PageTransition::GENERATED)
     return curr_instance;
 
   const GURL& dest_url = entry.url();
 
   // If we haven't used our SiteInstance (and thus RVH) yet, then we can use it
   // for this entry.  We won't commit the SiteInstance to this site until the
   // navigation commits (in DidNavigate), unless the navigation entry was
-  // restored. As session restore loads all the pages immediately we need to set
-  // the site first, otherwise after a restore none of the pages would share
-  // renderers.
+  // restored or it's a DOM UI as described below.
   if (!curr_instance->has_site()) {
     // If we've already created a SiteInstance for our destination, we don't
     // want to use this unused SiteInstance; use the existing one.  (We don't
     // do this check if the curr_instance has a site, because for now, we want
     // to compare against the current URL and not the SiteInstance's site.  In
     // this case, there is no current URL, so comparing against the site is ok.
     // See additional comments below.)
     if (curr_instance->HasRelatedSiteInstance(dest_url)) {
       return curr_instance->GetRelatedSiteInstance(dest_url);
     } else {
-      if (entry.restore_type() != NavigationEntry::RESTORE_NONE)
+      // Normally the "site" on the SiteInstance is set lazily when the load
+      // actually commits. This is to support better process sharing in case
+      // the site redirects to some other site: we want to use the destination
+      // site in the site instance.
+      //
+      // In the case of session restore, as it loads all the pages immediately
+      // we need to set the site first, otherwise after a restore none of the
+      // pages would share renderers.
+      //
+      // For DOM UI (this mostly comes up for the new tab page), the
+      // SiteInstance has special meaning: we never want to reassign the
+      // process. If you navigate to another site before the DOM UI commits,
+      // we still want to create a new process rather than re-using the
+      // existing DOM UI process.
+      if (entry.restore_type() != NavigationEntry::RESTORE_NONE ||
+          DOMUIFactory::HasDOMUIScheme(dest_url))
         curr_instance->SetSite(dest_url);
       return curr_instance;
     }
   }
 
   // Otherwise, only create a new SiteInstance for cross-site navigation.
 
   // TODO(creis): Once we intercept links and script-based navigations, we
   // will be able to enforce that all entries in a SiteInstance actually have
   // the same site, and it will be safe to compare the URL against the
@@ skipping 253 matching lines @@
   // deleted (not sure from where) but not NULLed.
   if (rvh == pending_render_view_host_) {
     // If you hit this NOTREACHED, please report it in the following bug
     // http://crbug.com/23411 Make sure to include what you were doing when it
     // happened  (navigating to a new page, closing a tab...) and if you can
     // reproduce.
     NOTREACHED();
     pending_render_view_host_ = NULL;
   }
 }