Match window's behavior when recursively copying directories that exist.

      Unit test improvement from here (sgk): http://codereview.chromium.org/271060

BUG=none
TEST=unittests

Review URL: http://codereview.chromium.org/270086

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@28838 0039d316-1c4b-4281-b951-d872f2087c98