Rename mBuf to mSlot in BufferItem

- All instances of mBuf have now been removed from the code base, so
  remove the union and get rid of it.

Bug 19769719

Change-Id: I49db71a664702dd355bdb9efe6b955fee2d54531
diff --git a/include/gui/BufferItem.h b/include/gui/BufferItem.h
index 145efe6..504ac64 100644
--- a/include/gui/BufferItem.h
+++ b/include/gui/BufferItem.h
@@ -98,13 +98,8 @@
         };
     };
 
-    union {
-        // mSlot is the slot index of this buffer (default INVALID_BUFFER_SLOT).
-        int mSlot;
-
-        // mBuf is the former name for mSlot
-        int mBuf;
-    };
+    // mSlot is the slot index of this buffer (default INVALID_BUFFER_SLOT).
+    int mSlot;
 
     // mIsDroppable whether this buffer was queued with the
     // property that it can be replaced by a new buffer for the purpose of