Disable layout test helper on windows

Disable the layout_test_helper binary on windows as it is only used to
control font smoothing, a task now handled by content_shell directly.

R=dpranke@chromium.org
BUG=356780

Review URL: https://codereview.chromium.org/226693008

git-svn-id: svn://svn.chromium.org/blink/trunk@171001 bbb929c8-8fbe-4397-9dbb-9b2b20218538