Protect GltfSchemaCommon classes from obfusctaion

Manually cherry-picked from go/tbcl/118984

Change-Id: Id000b0157da65c4747f6bff59209ed2bbbf61610

UnitySDK/Assets/TiltBrush/Scripts/Gltf/GltfSchemaCommon.cs

@@ -146,6 +146,7 @@ public class MeshPrecursor {
 // For more info on the spec, see:
 //   https://github.com/KhronosGroup/glTF/
 
+[Serializable]
 public abstract class GltfRootBase : IDisposable {
   public GltfAsset asset;
   // Tilt Brush/Blocks version that generated the gltf; null if not generated by that program
@@ -170,12 +171,14 @@ public void Dispose() {
   protected virtual void Dispose(bool disposing) {}
 }
 
+[Serializable]
 public abstract class GltfBufferBase {
   public long byteLength;
   public string uri;
   [JsonIgnore] public IBufferReader data;
 }
 
+[Serializable]
 public abstract class GltfAccessorBase {
   [Serializable] public enum ComponentType {
     BYTE = 5120, UNSIGNED_BYTE = 5121, SHORT = 5122, UNSIGNED_SHORT = 5123,
@@ -193,6 +196,7 @@ [Serializable] public enum ComponentType {
   public abstract GltfBufferViewBase BufferViewPtr { get; }
 }
 
+[Serializable]
 public abstract class GltfBufferViewBase {
   public int byteLength;
   public int byteOffset;
@@ -201,6 +205,7 @@ public abstract class GltfBufferViewBase {
   public abstract GltfBufferBase BufferPtr { get; }
 }
 
+[Serializable]
 public abstract class GltfPrimitiveBase {
   [Serializable] public enum Mode { TRIANGLES = 4 }
   public Mode mode = Mode.TRIANGLES;
@@ -220,25 +225,29 @@ public abstract class GltfPrimitiveBase {
   public abstract HashSet<string> GetAttributeNames();
 }
 
+[Serializable]
 public abstract class GltfImageBase {
   public string uri;
   [JsonIgnore] public RawImage data;
 }
 
+[Serializable]
 public abstract class GltfTextureBase {
   [JsonIgnore] public Texture2D unityTexture;
 
   public abstract object GltfId { get; }
   public abstract GltfImageBase SourcePtr { get; }
 }
 
+[Serializable]
 public abstract class GltfMaterialBase {
   public string name;
 
   public abstract Dictionary<string,string> TechniqueExtras { get; }
   public abstract IEnumerable<GltfTextureBase> ReferencedTextures { get; }
 }
 
+[Serializable]
 public abstract class GltfMeshBase {
   public string name;
 
@@ -247,6 +256,7 @@ public abstract class GltfMeshBase {
   public abstract GltfPrimitiveBase GetPrimitiveAt(int i);
 }
 
+[Serializable]
 public abstract class GltfNodeBase {
   public string name;
   public Matrix4x4? matrix;
@@ -256,6 +266,7 @@ public abstract class GltfNodeBase {
   public abstract IEnumerable<GltfNodeBase> Children { get; }
 }
 
+[Serializable]
 public abstract class GltfSceneBase {
   public Dictionary<string, string> extras;