Add Notification.maxActions so websites know how many buttons we support

public/platform/modules/notifications/WebNotificationManager.h

Index: public/platform/modules/notifications/WebNotificationManager.h
diff --git a/public/platform/modules/notifications/WebNotificationManager.h b/public/platform/modules/notifications/WebNotificationManager.h
index 9782331e6133abddb92cdf483538f365f88e8896..ed9ee2964283d2f68d052500db0362803fe2a8ea 100644
--- a/public/platform/modules/notifications/WebNotificationManager.h
+++ b/public/platform/modules/notifications/WebNotificationManager.h
@@ -58,6 +58,10 @@ public:
 
     // Synchronously checks the permission level for the given origin.
     virtual WebNotificationPermission checkPermission(const WebSecurityOrigin&) = 0;
+
+    // Returns the maximum number of actions supported by the embedder.
+    // TODO(johnme): Make pure virtual.
+    virtual size_t maxActions() { return 0; }
 };
 
 } // namespace blink