Add new experiment: frame_context_signaling.

This stores frame contexts alongside a reference frame, and always
uses the frame in reference slot 0 (LAST_FRAME) as the source of
the frame context.

The encoder could then reorder reference frames as to control
which frame context is used, however currently it does not.

Low Latency AWCY result:

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.1438 |  0.4161 |     N/A |   0.0386 | -0.0281 |  0.0453 |     0.2514

https://arewecompressedyet.com/?job=before-frame-context-signaling%402017-06-07T23%3A20%3A49.473Z&job=after-frame-context-signaling%402017-06-07T23%3A21%3A36.117Z

Change-Id: I4f6f9b12cb403573efbf9e5c3077d62f5dedc467
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 70316be..9d76628 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -4399,9 +4399,9 @@
     aom_wb_write_bit(
         wb, cm->refresh_frame_context == REFRESH_FRAME_CONTEXT_FORWARD);
   }
-
+#if !CONFIG_NO_FRAME_CONTEXT_SIGNALING
   aom_wb_write_literal(wb, cm->frame_context_idx, FRAME_CONTEXTS_LOG2);
-
+#endif
   assert(cm->mib_size == mi_size_wide[cm->sb_size]);
   assert(cm->mib_size == 1 << cm->mib_size_log2);
 #if CONFIG_EXT_PARTITION