Add spec links for WebGL IDL files

third_party/WebKit/Source/modules/webgl/WebGLCompressedTextureETC1.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// https://www.khronos.org/registry/webgl/extensions/WEBGL_compressed_texture_etc1/
+
 [
     DependentLifetime,
     NoInterfaceObject,
     DoNotCheckConstants,
 ] interface WebGLCompressedTextureETC1 {
     /* Compressed Texture Formats */
     const unsigned long COMPRESSED_RGB_ETC1_WEBGL = 0x8D64;
 };