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/BufferQueueConsumer.h b/include/gui/BufferQueueConsumer.h
index a9fce1a..8ec0546 100644
--- a/include/gui/BufferQueueConsumer.h
+++ b/include/gui/BufferQueueConsumer.h
@@ -140,6 +140,9 @@
     virtual status_t getOccupancyHistory(bool forceFlush,
             std::vector<OccupancyTracker::Segment>* outHistory) override;
 
+    // See IGraphicBufferConsumer::discardFreeBuffers
+    virtual status_t discardFreeBuffers() override;
+
     // dump our state in a String
     virtual void dump(String8& result, const char* prefix) const;