Use shutil.rmtree's 'onerror' error handling parameter to catch and handle th...

Use shutil.rmtree's 'onerror' error handling parameter to catch and handle the error that occurs when it's used on a symlink.  Ideally, we would just check os.path.islink as the code does for Linux/Mac, but os.path.islink doesn't work properly in python 2.x on Windows.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=1285
TEST=Tested on manually on the bot as follows:
   > mkdir test
   > mkdir test\1
   > mklink /D test\current test\1
   > python -c "import chromium_utils; chromium_utils.RemoveDirectory('test')"


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=70432

[dmichael(do not use this one), 2011-01-04 18:11:58]

This supersedes the CL I did that attempted to use os.path.islink, which didn't
work since os.path.islink always returns False in Python 2.x on Windows:
http://codereview.chromium.org/5975009/
http://codereview.chromium.org/6068003/

(That change apparently didn't do anything, since the svn repo got moved from
trunk/tools/buildbot/
to
trunk/tools/build/)

[nsylvain, 2011-01-04 19:06:19]

LGTM

[bradn, 2011-01-04 19:06:28]

otherwise LGTM