Add spec links for WebGL IDL files

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

 // Copyright 2016 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_etc/
+
 [
     DependentLifetime,
     NoInterfaceObject,
     DoNotCheckConstants,
 ] interface WebGLCompressedTextureETC {
     /* Compressed Texture Format */
     const unsigned long COMPRESSED_R11_EAC                        = 0x9270;
     const unsigned long COMPRESSED_SIGNED_R11_EAC                 = 0x9271;
     const unsigned long COMPRESSED_RG11_EAC                       = 0x9272;
     const unsigned long COMPRESSED_SIGNED_RG11_EAC                = 0x9273;
     const unsigned long COMPRESSED_RGB8_ETC2                      = 0x9274;
     const unsigned long COMPRESSED_SRGB8_ETC2                     = 0x9275;
     const unsigned long COMPRESSED_RGB8_PUNCHTHROUGH_ALPHA1_ETC2  = 0x9276;
     const unsigned long COMPRESSED_SRGB8_PUNCHTHROUGH_ALPHA1_ETC2 = 0x9277;
     const unsigned long COMPRESSED_RGBA8_ETC2_EAC                 = 0x9278;
     const unsigned long COMPRESSED_SRGB8_ALPHA8_ETC2_EAC          = 0x9279;
 };