wtf: Simplify OS(WINDOWS) check.

Stop checking of OS(WINDOWS) while already inside an OS(WINDOWS) block.

R=thakis@chromium.org,jochen@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19211002

git-svn-id: svn://svn.chromium.org/blink/trunk@154235 bbb929c8-8fbe-4397-9dbb-9b2b20218538