Move HdrMetadata to ColorSpace

Move HdrMetadata to ColorSpace as part of preparing for joint transmission
of these two objects.

Bug: webrtc:8651
Change-Id: Ie948011a2c0106d5967cb5ef3b9565217e798272
Reviewed-on: https://webrtc-review.googlesource.com/c/111481
Commit-Queue: Johannes Kron <kron@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25730}

modules/video_coding/codecs/vp9/vp9_impl.h

@@ -170,7 +170,7 @@ class VP9DecoderImpl : public VP9Decoder {
                   uint32_t timestamp,
                   int64_t ntp_time_ms,
                   int qp,
-                  const HdrMetadata* hdr_metadata);
+                  const ColorSpace* explicit_color_space);
 
   // Memory pool used to share buffers between libvpx and webrtc.
   Vp9FrameBufferPool frame_buffer_pool_;