fix: set yuv full range flag to 1 for VP9 with sRGB (shaka-project#1398)

If color_space is VPX_COLOR_SPACE_SRGB, the specs says that color_range
should be 1 i.e. yuv_full_range = true. 

However, yuv_full_range was initialized as false and wasn't set in the branch for color_space
is VPX_COLOR_SPACE_SRGB.

Fixes shaka-project#990

---------

Co-authored-by: Joey Parrish <joeyparrish@users.noreply.github.com>

packager/media/codecs/vp9_parser.cc

@@ -285,6 +285,7 @@ bool ReadBitDepthAndColorSpace(BitReader* reader,
     }
   } else {
     // Assume 4:4:4 for colorspace SRGB.
+    yuv_full_range = true;
     chroma_subsampling = VPCodecConfigurationRecord::CHROMA_444;
     if (codec_config->profile() & 1) {
       bool reserved;