BufferQueue consumers: Add discardFreeBuffer method

This method releases all free buffers owned by the buffer queue,
in order to save memory (at the cost of potential future
reallocation of buffers).

Bug: 28695173
Change-Id: I458d10373e639e3144faf673af2ba01aca36e65a
diff --git a/include/gui/ConsumerBase.h b/include/gui/ConsumerBase.h
index d1f4cdd..0490c3c 100644
--- a/include/gui/ConsumerBase.h
+++ b/include/gui/ConsumerBase.h
@@ -89,6 +89,9 @@
     status_t getOccupancyHistory(bool forceFlush,
             std::vector<OccupancyTracker::Segment>* outHistory);
 
+    // See IGraphicBufferConsumer::discardFreeBuffers
+    status_t discardFreeBuffers();
+
 private:
     ConsumerBase(const ConsumerBase&);
     void operator=(const ConsumerBase&);