Skip Direct Write tests on XP

Skip the virtual/windows-directwrite suite on Windows XP as Direct
Write is not supported on that platform.

TBR=dglazkov@chromium.org
BUG=356780

Review URL: https://codereview.chromium.org/236593008

git-svn-id: svn://svn.chromium.org/blink/trunk@171505 bbb929c8-8fbe-4397-9dbb-9b2b20218538