Support window.opener after a process swap.

content/browser/site_instance.h

Index: content/browser/site_instance.h
diff --git a/content/browser/site_instance.h b/content/browser/site_instance.h
index 76f966eafa72d7b4d390459154d6270182bee974..3b6346d5ea367c7dbbab2dff7995d2e5c201bf59 100644
--- a/content/browser/site_instance.h
+++ b/content/browser/site_instance.h
@@ -50,6 +50,9 @@ class BrowsingInstance;
 class SiteInstance : public base::RefCounted<SiteInstance>,
                      public NotificationObserver {
  public:
+  // Returns a unique ID for this SiteInstance.
+  int32 id() { return id_; }
+
   // Get the BrowsingInstance to which this SiteInstance belongs.
   BrowsingInstance* browsing_instance() { return browsing_instance_; }
 
@@ -162,6 +165,12 @@ class SiteInstance : public base::RefCounted<SiteInstance>,
                        const NotificationSource& source,
                        const NotificationDetails& details);
 
+  // The next available SiteInstance ID.
+  static int32 next_site_instance_id_;
+
+  // A unique ID for this SiteInstance.
+  int32 id_;
+
   NotificationRegistrar registrar_;
 
   // BrowsingInstance to which this SiteInstance belongs.