fix an issue where invalidate/transactions could be missed

Change-Id: I84a1fcba1317b2631f5441de7b7ecd12af5ad022
diff --git a/services/surfaceflinger/MessageQueue.h b/services/surfaceflinger/MessageQueue.h
index 775400f..2317d81 100644
--- a/services/surfaceflinger/MessageQueue.h
+++ b/services/surfaceflinger/MessageQueue.h
@@ -55,6 +55,7 @@
 
 class MessageQueue {
     sp<Looper> mLooper;
+    volatile int32_t mWorkPending;
 
 public:
     MessageQueue();