glb2: Don't warn about gltf2 + Tilt Brush

It's now normal and expected to see Gltf2Materials in Tilt Brush files.
There's no way to get at shader properties, but that's fine -- with
gltf2, Tilt Brush can use PBR to export non-brush-related geometry
rather than use the old "template" BrushDescriptor trick.

Change-Id: Id000b0154581f86cf625d022ae75c90373decaff

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

@@ -426,6 +426,8 @@ private static Guid ParseGuidFromShader(Gltf1Material material) {
   /// Returns a BrushDescriptor given a gltf material, or null if not found.
   /// If the material is an instance of a template, the descriptor for that
   /// will be returned.
+  /// Note that gltf2 has pbr support, and Tilt Brush uses that instead of
+  /// template "brushes".
   public static BrushDescriptor LookupBrushDescriptor(GltfMaterialBase gltfMaterial) {
     Guid guid = ParseGuidFromMaterial(gltfMaterial);
     if (guid == Guid.Empty) {
@@ -439,7 +441,6 @@ public static BrushDescriptor LookupBrushDescriptor(GltfMaterialBase gltfMateria
         // can be found on the shader.
         Gltf1Material gltf1Material = gltfMaterial as Gltf1Material;
         if (gltf1Material == null) {
-          Debug.LogErrorFormat("Unexpected: glTF2 Tilt Brush material");
           return null;
         }
         Guid templateGuid = ParseGuidFromShader((Gltf1Material)gltfMaterial);