libgui: Add getLastQueuedBuffer to BufferQueue

Adds the ability to get the last buffer queued to a BufferQueue plus
its acquire fence.

Bug: 27708453
Change-Id: Iee39475740b40c854a5f46178b2934fd930e61b8
diff --git a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h
index 818d3eb..bd1cfb1 100644
--- a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h
+++ b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h
@@ -126,6 +126,8 @@
     virtual status_t setSharedBufferMode(bool sharedBufferMode) override;
     virtual status_t setAutoRefresh(bool autoRefresh) override;
     virtual status_t setDequeueTimeout(nsecs_t timeout) override;
+    virtual status_t getLastQueuedBuffer(sp<GraphicBuffer>* outBuffer,
+            sp<Fence>* outFence) override;
 
     //
     // Utility methods