Roll buildtools and clang for GN build.

DEPS

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 vars = {
   # The dart_root is the root of our sdk checkout. This is normally
   # simply sdk, but if using special gclient specs it can be different.
   "dart_root": "sdk",
 
   # We use mirrors of all github repos to guarantee reproducibility and
@@ skipping 13 matching lines @@
   # Chromium git
   "chromium_git": "https://chromium.googlesource.com",
 
   # Only use this temporarily while waiting for a mirror for a new package.
   "github_dartlang": "https://github.com/dart-lang/%s.git",
 
   "gyp_rev": "@6ee91ad8659871916f9aa840d42e1513befdf638",
   "co19_rev": "@d4767b4caea3c5828ad8e053cd051d44a59061af",
 
   # Revisions of GN related dependencies.
-  "buildtools_revision": "@3d2e47bf14e4e67816a53e304dea422fa18f9180",
+  "buildtools_revision": "@39b1db2ab4aa4b2ccaa263c29bdf63e7c1ee28aa",
 
   "gperftools_revision": "@7822b5b0b9fa7e016e1f6b46ea86f26f4691a457",
 
   # Revisions of /third_party/* dependencies.
   "args_tag": "@0.13.5",
   "async_tag": "@1.11.2",
   "barback-0.13.0_rev": "@34853",
   "barback-0.14.0_rev": "@36398",
   "barback-0.14.1_rev": "@38525",
   "barback_tag" : "@0.15.2+9",
@@ skipping 520 matching lines @@
     # Update the Windows toolchain if necessary.
     'name': 'win_toolchain',
     'pattern': '.',
     'action': ['python', 'sdk/build/vs_toolchain.py', 'update'],
   },
   {
     "pattern": ".",
     "action": ["python", Var("dart_root") + "/tools/generate_buildfiles.py"],
   },
 ]