native: Rename mBuf to mSlot

Remove the union in BufferItem.

Bug: 19769719
Change-Id: I4e496b5aafce0fc5206dc7551a741f9c262c2cd0
diff --git a/libs/gui/StreamSplitter.cpp b/libs/gui/StreamSplitter.cpp
index 43f9214..dce0e87 100644
--- a/libs/gui/StreamSplitter.cpp
+++ b/libs/gui/StreamSplitter.cpp
@@ -132,7 +132,7 @@
     ALOGV("acquired buffer %#" PRIx64 " from input",
             bufferItem.mGraphicBuffer->getId());
 
-    status = mInput->detachBuffer(bufferItem.mBuf);
+    status = mInput->detachBuffer(bufferItem.mSlot);
     LOG_ALWAYS_FATAL_IF(status != NO_ERROR,
             "detaching buffer from input failed (%d)", status);