add a mechanism to define clang tarball mirrors

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 52 matching lines @@
 LLVM_BUILD_TOOLS_DIR = os.path.abspath(
     os.path.join(LLVM_DIR, '..', 'llvm-build-tools'))
 STAMP_FILE = os.path.normpath(
     os.path.join(LLVM_DIR, '..', 'llvm-build', 'cr_build_revision'))
 BINUTILS_DIR = os.path.join(THIRD_PARTY_DIR, 'binutils')
 VERSION = '3.9.0'
 ANDROID_NDK_DIR = os.path.join(
     CHROMIUM_DIR, 'third_party', 'android_tools', 'ndk')
 
 # URL for pre-built binaries.
-CDS_URL = 'https://commondatastorage.googleapis.com/chromium-browser-clang'
+CDS_URL = os.environ.get('CDS_CLANG_BUCKET_OVERRIDE',
+    'https://commondatastorage.googleapis.com/chromium-browser-clang')
 
 LLVM_REPO_URL='https://llvm.org/svn/llvm-project'
 if 'LLVM_REPO_URL' in os.environ:
   LLVM_REPO_URL = os.environ['LLVM_REPO_URL']
 
 
 def DownloadUrl(url, output_file):
   """Download url into output_file."""
   CHUNK_SIZE = 4096
   TOTAL_DOTS = 10
@@ skipping 679 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())