Fix searching for DirectX SDK during GN build.

Before that GN just checked for DXSDK_DIR environment variable.
GYP does more and checks registry, let's do the same in GN.

R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/37599004

Patch from Vyacheslav Chigrin <vchigrin@yandex-team.ru>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@8066 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/build/find_directx_sdk.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python
+#
+# Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
+#
+# Use of this source code is governed by a BSD-style license
+# that can be found in the LICENSE file in the root of the source
+# tree. An additional intellectual property rights grant can be found
+# in the file PATENTS.  All contributing project authors may
+# be found in the AUTHORS file in the root of the source tree.
+
+"""Searches for DirectX SDK installation and prints full path to it."""
+
+import os
+import subprocess
+import sys
+
+
+def main():
+  sys.stdout.write(FindDirectXInstallation())
+  return 0
+
+
+def FindDirectXInstallation():
+  """Try to find an installation location for the DirectX SDK. Check for the
+  standard environment variable, and if that doesn't exist, try to find
+  via the registry. Returns empty string if not found in either location."""
+
+  dxsdk_dir = os.environ.get('DXSDK_DIR')
+  if dxsdk_dir:
+    return dxsdk_dir
+
+  # Setup params to pass to and attempt to launch reg.exe.
+  cmd = ['reg.exe', 'query', r'HKLM\Software\Microsoft\DirectX', '/s']
+  p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+  for line in p.communicate()[0].splitlines():
+    if 'InstallPath' in line:
+      return line.split('    ')[3] + "\\"
+
+  return ''
+
+
+if __name__ == '__main__':
+  sys.exit(main())