Port //components/webp_transcode to build with gn.

components/webp_transcode.gypi

 # Copyright 2013 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.
 
 {
   'targets': [
     {
+      # GN version: //components/webp_transcode
       'target_name': 'webp_transcode',
       'type': 'static_library',
       'dependencies': [
         '../base/base.gyp:base',
         '../ios/net/ios_net.gyp:ios_net',
         '../net/net.gyp:net',
         '../third_party/libwebp/libwebp.gyp:libwebp_dec',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
         'webp_transcode/webp_decoder.h',
         'webp_transcode/webp_decoder.mm',
         'webp_transcode/webp_network_client.h',
         'webp_transcode/webp_network_client.mm',
         'webp_transcode/webp_network_client_factory.h',
         'webp_transcode/webp_network_client_factory.mm',
        ],
     },
   ],
 }