native: Rename mBuf to mSlot

Remove the union in BufferItem.

Bug: 19769719
Change-Id: I4e496b5aafce0fc5206dc7551a741f9c262c2cd0
diff --git a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.cpp b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.cpp
index ba4c198..470f153 100644
--- a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.cpp
+++ b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.cpp
@@ -442,11 +442,11 @@
         result = acquireBufferLocked(&item, 0);
         if (result != NO_ERROR)
             return result;
-        VDS_LOGW_IF(item.mBuf != sslot,
+        VDS_LOGW_IF(item.mSlot != sslot,
                 "queueBuffer: acquired sslot %d from SCRATCH after queueing sslot %d",
-                item.mBuf, sslot);
-        mFbProducerSlot = mapSource2ProducerSlot(SOURCE_SCRATCH, item.mBuf);
-        mFbFence = mSlots[item.mBuf].mFence;
+                item.mSlot, sslot);
+        mFbProducerSlot = mapSource2ProducerSlot(SOURCE_SCRATCH, item.mSlot);
+        mFbFence = mSlots[item.mSlot].mFence;
 
     } else {
         LOG_FATAL_IF(mCompositionType != COMPOSITION_GLES,