Merge "libgui: Add a union to BufferItem for refactoring"

    // mFrameNumber is the number of the queued frame for this slot.

    uint64_t mFrameNumber;

    union {

        // mSlot is the slot index of this buffer (default INVALID_BUFFER_SLOT).

        int mSlot;

        // mBuf is the former name for mSlot

        int mBuf;

    };

    // mIsDroppable whether this buffer was queued with the

    // property that it can be replaced by a new buffer for the purpose of

    // making sure dequeueBuffer() won't block.