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..bd86cbdcb4106ab52129eb68801581ca476b2784 100644
--- a/public/platform/modules/notifications/WebNotificationManager.h
+++ b/public/platform/modules/notifications/WebNotificationManager.h
@@ -58,6 +58,9 @@ public:
 
     // Synchronously checks the permission level for the given origin.
     virtual WebNotificationPermission checkPermission(const WebSecurityOrigin&) = 0;
+
+    // Returns the maximum number of action buttons supported by the embedder.

[Peter Beverloo, 2015/07/31 12:28:58]

s/action buttons/actions/

+    virtual size_t maxActions() { return 0; } // TODO(johnme): Make pure virtual.

[Peter Beverloo, 2015/07/31 12:28:58]

nit: Please place the TODO on the line above.

 };
 
 } // namespace blink