Implement PermissionService::GetNextPermissionChange.

content/public/browser/content_browser_client.h

Index: content/public/browser/content_browser_client.h
diff --git a/content/public/browser/content_browser_client.h b/content/public/browser/content_browser_client.h
index 31959739147ca2cb4c74666050db4a9c7f1c45ef..0b236a326a2c139dd74965c837c95bf638085487 100644
--- a/content/public/browser/content_browser_client.h
+++ b/content/public/browser/content_browser_client.h
@@ -453,6 +453,20 @@ class CONTENT_EXPORT ContentBrowserClient {
       const GURL& requesting_origin,
       const GURL& embedding_origin);
 
+  // Runs the given |callback| whenever the |permission| associated with the
+  // pair { requesting_origin, embedding_origin } changes.
+  // Returns the subscription_id to be used to unsubscribe.
+  virtual int SubscribePermissionStatusChange(
+      PermissionType permission,
+      BrowserContext* browser_context,
+      const GURL& requesting_origin,
+      const GURL& embedding_origin,
+      const base::Callback<void(PermissionStatus)>& callback);
+
+  virtual void UnsubscribePermissionStatusChange(
+      BrowserContext* browser_context,
+      int subscription_id) {}
+
   virtual void ResetPermission(PermissionType permission,
                                BrowserContext* browser_context,
                                const GURL& requesting_origin,