Add API hooks for setting chroma subsampling values.

BUG=aomedia:2107

Change-Id: Iac96b16b03c8b895463dfbf29feade799f458476
diff --git a/av1/encoder/encoder.h b/av1/encoder/encoder.h
index 3ba1354..a47a4d7 100644
--- a/av1/encoder/encoder.h
+++ b/av1/encoder/encoder.h
@@ -309,6 +309,9 @@
   float noise_level;
   int noise_block_size;
 #endif
+
+  unsigned int chroma_subsampling_x;
+  unsigned int chroma_subsampling_y;
 } AV1EncoderConfig;
 
 static INLINE int is_lossless_requested(const AV1EncoderConfig *cfg) {