[Cast] Make decoder_config.h usable without decoder_config_adapter.cc

chromecast/public/media/decoder_config.h

Index: chromecast/public/media/decoder_config.h
diff --git a/chromecast/public/media/decoder_config.h b/chromecast/public/media/decoder_config.h
index c493796dae4ed964f7ee35d4d3b7f06a2d209f0f..d4902984f5eb659e0df50f4b89f2225c9bcab058 100644
--- a/chromecast/public/media/decoder_config.h
+++ b/chromecast/public/media/decoder_config.h
@@ -287,6 +287,13 @@ struct HDRMetadata {
   HDRMetadata();
   HDRMetadata(const HDRMetadata& rhs);
 };
+
+inline MasteringMetadata::MasteringMetadata() {}
+inline MasteringMetadata::MasteringMetadata(const MasteringMetadata& rhs) =
+    default;
+
+inline HDRMetadata::HDRMetadata() {}
+inline HDRMetadata::HDRMetadata(const HDRMetadata& rhs) = default;
 // ---- End copy/paste from media/base/hdr_metadata.h ----
 
 // TODO(erickung): Remove constructor once CMA backend implementation doesn't