Revert of Remove depenedency on GYP_DEFINES in clang gclient hook

tools/clang/scripts/update.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """This script is used to download prebuilt clang binaries.
 
 It is also used by package.py to build the prebuilt clang binaries."""
 
 import argparse
@@ skipping 379 matching lines @@
   version_out = re.match(r'clang version ([0-9.]+)', version_out).group(1)
   if version_out != VERSION:
     print ('unexpected clang version %s (not %s), update VERSION in update.py'
            % (version_out, VERSION))
     sys.exit(1)
 
 
 def UpdateClang(args):
   print 'Updating Clang to %s...' % PACKAGE_VERSION
 
-  # Required for LTO, which is used when is_official_build = true.
-  need_gold_plugin = sys.platform.startswith('linux')
+  need_gold_plugin = 'LLVM_DOWNLOAD_GOLD_PLUGIN' in os.environ or (
+      sys.platform.startswith('linux') and
+      'buildtype=Official' in os.environ.get('GYP_DEFINES', ''))
 
   if ReadStampFile() == PACKAGE_VERSION and not args.force_local_build:
     print 'Clang is already up to date.'
     if not need_gold_plugin or os.path.exists(
         os.path.join(LLVM_BUILD_DIR, "lib/LLVMgold.so")):
       return 0
 
   # Reset the stamp file in case the build is unsuccessful.
   WriteStampFile('')
 
@@ skipping 480 matching lines @@
     args.force_local_build = True
     if 'OS=android' not in os.environ.get('GYP_DEFINES', ''):
       # Only build the Android ASan rt on ToT bots when targetting Android.
       args.with_android = False
 
   return UpdateClang(args)
 
 
 if __name__ == '__main__':
   sys.exit(main())