Add support for Rec709 color space videos in software YUV convert path.

media/base/yuv_convert.h

Index: media/base/yuv_convert.h
diff --git a/media/base/yuv_convert.h b/media/base/yuv_convert.h
index cf13edb17ab285ff5e74e52e2a26fce2f7a5cdff..8bb68831eac3b9b4b15c9d352d334d952b376059 100644
--- a/media/base/yuv_convert.h
+++ b/media/base/yuv_convert.h
@@ -27,6 +27,7 @@ enum YUVType {
   YV16  = 0,  // YV16 is half width and full height chroma channels.
   YV12  = 1,  // YV12 is half width and half height chroma channels.
   YV12J = 2,  // YV12J is the same as YV12, but in JPEG color range.
+  YV12HD = 3,  // YV12HD is the same as YV12, but in 'HD' Rec709 color space.
 };
 
 // Get the appropriate value to bitshift by for vertical indices.