Make Clang's update.py use the Win toolchain under depot_tools if available

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.
 
 """Windows can't run .sh files, so this is a Python implementation of
 update.sh. This script should replace update.sh on all platforms eventually."""
 
 import os
 import re
@@ skipping 118 matching lines @@
     if os.path.isdir(d):
       os.environ['PATH'] = os.environ.get('PATH', '') + os.pathsep + d
       return
   print 'Failed to find CMake!'
   sys.exit(1)
 
 
 vs_version = None
 def GetVSVersion():
   global vs_version
-  if not vs_version:
-    # TODO(hans): Find a less hacky way to find the MSVS installation.
-    sys.path.append(os.path.join(CHROMIUM_DIR, 'tools', 'gyp', 'pylib'))
-    import gyp.MSVSVersion
-    # We request VS 2013 because Clang won't build with 2010, and 2013 will be
-    # the default for Chromium soon anyway.
-    vs_version = gyp.MSVSVersion.SelectVisualStudioVersion('2013')
+  if vs_version:
+    return vs_version
+
+  # Try using the toolchain in depot_tools.
+  # This sets environment variables used by SelectVisualStudioVersion below.
+  sys.path.append(os.path.join(CHROMIUM_DIR, 'build'))
+  import vs_toolchain
+  vs_toolchain.SetEnvironmentAndGetRuntimeDllDirs()
+
+  # Use gyp to find the MSVS installation, either in depot_tools as per above,
+  # or a system-wide installation otherwise.
+  sys.path.append(os.path.join(CHROMIUM_DIR, 'tools', 'gyp', 'pylib'))
+  import gyp.MSVSVersion
+  vs_version = gyp.MSVSVersion.SelectVisualStudioVersion('2013')
   return vs_version
 
 
 def UpdateClang():
   print 'Updating Clang to %s...' % (LLVM_WIN_REVISION)
   if LLVM_WIN_REVISION != 'HEAD' and ReadStampFile() == LLVM_WIN_REVISION:
     print 'Already up to date.'
     return 0
 
   AddCMakeToPath()
@@ skipping 84 matching lines @@
 
   if re.search(r'\b(make_clang_dir)=', os.environ.get('GYP_DEFINES', '')):
     print 'Skipping Clang update (make_clang_dir= was set in GYP_DEFINES).'
     return 0
 
   return UpdateClang()
 
 
 if __name__ == '__main__':
   sys.exit(main())