native: Rename mBuf to mSlot

Remove the union in BufferItem.

Bug: 19769719
Change-Id: I4e496b5aafce0fc5206dc7551a741f9c262c2cd0
diff --git a/libs/gui/tests/BufferQueue_test.cpp b/libs/gui/tests/BufferQueue_test.cpp
index 1a54875..deec68c 100644
--- a/libs/gui/tests/BufferQueue_test.cpp
+++ b/libs/gui/tests/BufferQueue_test.cpp
@@ -288,8 +288,8 @@
     BufferItem item;
     ASSERT_EQ(OK, mConsumer->acquireBuffer(&item, static_cast<nsecs_t>(0)));
 
-    ASSERT_EQ(OK, mConsumer->detachBuffer(item.mBuf));
-    ASSERT_EQ(BAD_VALUE, mConsumer->detachBuffer(item.mBuf)); // Not acquired
+    ASSERT_EQ(OK, mConsumer->detachBuffer(item.mSlot));
+    ASSERT_EQ(BAD_VALUE, mConsumer->detachBuffer(item.mSlot)); // Not acquired
 
     uint32_t* dataIn;
     ASSERT_EQ(OK, item.mGraphicBuffer->lock(
@@ -348,7 +348,7 @@
 
     BufferItem item;
     ASSERT_EQ(OK, mConsumer->acquireBuffer(&item, static_cast<nsecs_t>(0)));
-    ASSERT_EQ(OK, mConsumer->detachBuffer(item.mBuf));
+    ASSERT_EQ(OK, mConsumer->detachBuffer(item.mSlot));
 
     int newSlot;
     ASSERT_EQ(OK, mProducer->attachBuffer(&newSlot, item.mGraphicBuffer));