Set VPX_FRAME_IS_DROPPABLE

Allow the encoder to inform the application that the encoded frame will not
be used as a reference.

Change-Id: I90e41962325ef73d44da03327deb340d6f7f4860
diff --git a/vp8/vp8_cx_iface.c b/vp8/vp8_cx_iface.c
index 5e1278a..966355d 100644
--- a/vp8/vp8_cx_iface.c
+++ b/vp8/vp8_cx_iface.c
@@ -798,6 +798,9 @@
                     pkt.data.frame.duration = 0;
                 }
 
+                if (cpi->droppable)
+                    pkt.data.frame.flags |= VPX_FRAME_IS_DROPPABLE;
+
                 if (cpi->output_partition)
                 {
                     int i;