Fix webkit_unit_tests win static library compile.

Same thing as in https://code.google.com/p/chromium/issues/detail?id=310636


BUG=

Review URL: https://codereview.chromium.org/58833005

git-svn-id: svn://svn.chromium.org/blink/trunk@161366 bbb929c8-8fbe-4397-9dbb-9b2b20218538