Don't perform a microtask checkpoint for document.write('<script><\/script>')

This is a willful violation of the spec, expecting that it will change:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=24361

BUG=340322

Review URL: https://codereview.chromium.org/169093004

git-svn-id: svn://svn.chromium.org/blink/trunk@167302 bbb929c8-8fbe-4397-9dbb-9b2b20218538