[Extensions] Propagate activeTab hosts to extension background pages

extensions/browser/process_manager.h

Index: extensions/browser/process_manager.h
diff --git a/extensions/browser/process_manager.h b/extensions/browser/process_manager.h
index 658fcd38612f3398922a5f258ce12dc1a0a67a3e..2bfcb60e4d2f587b039ff6a62ea10bb1a3eabeed 100644
--- a/extensions/browser/process_manager.h
+++ b/extensions/browser/process_manager.h
@@ -73,6 +73,10 @@ class ProcessManager : public KeyedService,
   ExtensionHost* GetBackgroundHostForExtension(const std::string& extension_id);
 
   // Returns the SiteInstance that the given URL belongs to.
+  // Callers should wrap the result in a scoped_refptr to ensure the
+  // SiteInstance becomes refcounted.
+  // TODO(devlin): The above comment clearly indicates that this should just
+  // return a refptr. Update callers.
   // TODO(aa): This only returns correct results for extensions and packaged
   // apps, not hosted apps.
   virtual content::SiteInstance* GetSiteInstanceForURL(const GURL& url);