Force show_frame=1 when show_existing_frame=1

When showing a previously decoded frame, we need to
explicitly set the show_frame flag.

For the current frame being decoded this flag is
explicitly set in the frame header.

This should fix WebM Issue 696:
http://code.google.com/p/webm/issues/detail?id=696

Change-Id: I5751a809813f88d2ca6f62c47c3878475ff9ba8d

vp9/decoder/vp9_decodeframe.c

diff --git a/vp9/decoder/vp9_decodeframe.c b/vp9/decoder/vp9_decodeframe.c
index 59faccd..7b856d7 100644
--- a/vp9/decoder/vp9_decodeframe.c
+++ b/vp9/decoder/vp9_decodeframe.c
@@ -1129,11 +1129,12 @@
 
   cm->show_existing_frame = vp9_rb_read_bit(rb);
   if (cm->show_existing_frame) {
-    // show an existing frame directly
+    // Show an existing frame directly.
     int frame_to_show = cm->ref_frame_map[vp9_rb_read_literal(rb, 3)];
     ref_cnt_fb(cm->fb_idx_ref_cnt, &cm->new_fb_idx, frame_to_show);
     pbi->refresh_frame_flags = 0;
     cm->lf.filter_level = 0;
+    cm->show_frame = 1;
     return 0;
   }