Return fence from acquireBuffer

Change-Id: Iab22054c1dc4fd84affab3cc5bbdcd5a1e689666
diff --git a/include/gui/BufferQueue.h b/include/gui/BufferQueue.h
index 7c1fcb9..0d36baa 100644
--- a/include/gui/BufferQueue.h
+++ b/include/gui/BufferQueue.h
@@ -202,6 +202,9 @@
 
         // mBuf is the slot index of this buffer
         int mBuf;
+
+        // mFence is a fence that will signal when the buffer is idle.
+        sp<Fence> mFence;
     };
 
     // The following public functions is the consumer facing interface