Fix ninja_clobber on Windows.
Fix ninja_clobber on Windows.

Clobber failed like this
http://build.chromium.org/p/chromium.fyi/builders/Chromium%20Win7%20Ninja%20Goma/builds/1835/steps/compile/logs/stdio

 chdir to E:\b\build\slave\Chromium_Win7_Ninja_Goma\build\src
 Removing out\Release
 Traceback (most recent call last):
  File "..\..\..\scripts\slave\compile.py", line 1101, in <module>
    sys.exit(real_main())
  File "..\..\..\scripts\slave\compile.py", line 1097, in real_main
    return main(options, args)
  File "..\..\..\scripts\slave\compile.py", line 702, in main_ninja
    ninja_clobber(output_dir)
  File "..\..\..\scripts\slave\compile.py", line 118, in ninja_clobber
    os.rmdir(root)
 WindowsError: [Error 145] The directory is not empty: 'out\\Release\\obj\\ash'
Review URL: https://chromiumcodereview.appspot.com/10824176

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@150066 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build