avoid kWhatReadBuffer message spam in GenericSource

When playig WVM video, an extra readBuffer is posted for each
dequeueAccessUnit, resulting in an unbounded number of messages.

Bug: 17472979
Change-Id: Ice92ccf2454bf7a70856a4a8b535cefffc9e61ac
diff --git a/media/libmediaplayerservice/nuplayer/GenericSource.h b/media/libmediaplayerservice/nuplayer/GenericSource.h
index 50ff98a..c70c48e 100644
--- a/media/libmediaplayerservice/nuplayer/GenericSource.h
+++ b/media/libmediaplayerservice/nuplayer/GenericSource.h
@@ -131,6 +131,8 @@
     off64_t mMetaDataSize;
     int64_t mBitrate;
     int32_t mPollBufferingGeneration;
+    uint32_t mPendingReadBufferTypes;
+    mutable Mutex mReadBufferLock;
 
     sp<ALooper> mLooper;