Check BrowsingInstance before swapping prerenders.

content/public/browser/site_instance.h

Index: content/public/browser/site_instance.h
diff --git a/content/public/browser/site_instance.h b/content/public/browser/site_instance.h
index 2cede0abf411cf5c9cf22e2a61e9ea6c1a8781ad..66ed28839dbbb64744fa045df536c4a5b3b361d8 100644
--- a/content/public/browser/site_instance.h
+++ b/content/public/browser/site_instance.h
@@ -107,6 +107,10 @@ class CONTENT_EXPORT SiteInstance : public base::RefCounted<SiteInstance> {
   // origins (e.g., postMessage) should be supported.
   virtual bool IsRelatedSiteInstance(const SiteInstance* instance) = 0;
 
+  // Returns the total active WebContents count for this SiteInstance and all
+  // related SiteInstances in the same BrowsingInstance.
+  virtual size_t GetRelatedActiveContentsCount() const = 0;
+
   // Factory method to create a new SiteInstance.  This will create a new
   // new BrowsingInstance, so it should only be used when creating a new tab
   // from scratch (or similar circumstances).  Callers should ensure that