Check BrowsingInstance before swapping prerenders.

content/browser/site_instance_impl.h

Index: content/browser/site_instance_impl.h
diff --git a/content/browser/site_instance_impl.h b/content/browser/site_instance_impl.h
index edaeb3e702d75910effe7b4064ba20a3396ad84e..4824f77ea368588db987bbd30a3093471edf1037 100644
--- a/content/browser/site_instance_impl.h
+++ b/content/browser/site_instance_impl.h
@@ -21,10 +21,11 @@ class CONTENT_EXPORT SiteInstanceImpl : public SiteInstance,
   virtual int32 GetId() OVERRIDE;
   virtual bool HasProcess() const OVERRIDE;
   virtual RenderProcessHost* GetProcess() OVERRIDE;
+  virtual BrowserContext* GetBrowserContext() const OVERRIDE;
   virtual const GURL& GetSiteURL() const OVERRIDE;
   virtual SiteInstance* GetRelatedSiteInstance(const GURL& url) OVERRIDE;
   virtual bool IsRelatedSiteInstance(const SiteInstance* instance) OVERRIDE;
-  virtual BrowserContext* GetBrowserContext() const OVERRIDE;
+  virtual size_t GetRelatedActiveContentsCount() const OVERRIDE;
 
   // Set the web site that this SiteInstance is rendering pages for.
   // This includes the scheme and registered domain, but not the port.  If the
@@ -59,6 +60,9 @@ class CONTENT_EXPORT SiteInstanceImpl : public SiteInstance,
   // discarded to save memory.
   size_t active_view_count() { return active_view_count_; }

[davidben, 2014/04/29 17:01:25]

Oh, I mentioned this out-of-band, but the reason this value doesn't work is
because it also counts pending RVHs. The existing use of that count (when
swapped out RVHs can go away) should including pending RVHs because there may be
a race.

[Charlie Reis, 2014/04/29 17:31:08]

This count is also specific to the SiteInstance and not the BrowsingInstance,
right?  I agree that it wouldn't work.

[davidben, 2014/04/29 21:31:40]

Yeah. Though if that were the only issue, BrowsingInstance could maintain a set
of all SiteInstances attached to it and sum over the active_view_counts.

 
+  void IncrementRelatedActiveContentsCount();

[Charlie Reis, 2014/04/29 17:31:08]

These need comments explaining that they're for tracking the number of
WebContents in the BrowsingInstance, not counting pending hosts.

[davidben, 2014/04/29 21:31:40]

Done.

+  void DecrementRelatedActiveContentsCount();
+
   // Sets the global factory used to create new RenderProcessHosts.  It may be
   // NULL, in which case the default BrowserRenderProcessHost will be created
   // (this is the behavior if you don't call this function).  The factory must