Check BrowsingInstance before swapping prerenders.

content/browser/browsing_instance.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 "content/browser/browsing_instance.h"
 
 #include "base/command_line.h"
 #include "base/logging.h"
 #include "content/browser/site_instance_impl.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/content_browser_client.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/url_constants.h"
 
 namespace content {
 
 BrowsingInstance::BrowsingInstance(BrowserContext* browser_context)
-    : browser_context_(browser_context) {
+    : browser_context_(browser_context),
+      active_contents_count_(0u) {
 }
 
 bool BrowsingInstance::HasSiteInstance(const GURL& url) {
   std::string site =
       SiteInstanceImpl::GetSiteForURL(browser_context_, url)
           .possibly_invalid_spec();
 
   return site_instance_map_.find(site) != site_instance_map_.end();
 }
 
@@ skipping 48 matching lines @@
   if (i != site_instance_map_.end() && i->second == site_instance) {
     // Matches, so erase it.
     site_instance_map_.erase(i);
   }
 }
 
 BrowsingInstance::~BrowsingInstance() {
   // We should only be deleted when all of the SiteInstances that refer to
   // us are gone.
   DCHECK(site_instance_map_.empty());
+  DCHECK_EQ(0u, active_contents_count_);
 }
 
 }  // namespace content