Perform a microtask checkpoint for document.write('<script><\/script>')

http://whatwg.org/html#scriptEndTag

The spec does 'Perform a microtask checkpoint' unconditionally for 'An
end tag whose tag name is "script"', which means that document.write
can trigger it while a script is running.

A spec bug was filed for the similar situation with 'Provide a stable
state' on the assumption that this was a mistake, but it was not:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=24361

Unless performing a microtask checkpoint here will lead to other
complications, it's simpler to just align with the spec.

BUG=340322

Review URL: https://codereview.chromium.org/130983011

git-svn-id: svn://svn.chromium.org/blink/trunk@166445 bbb929c8-8fbe-4397-9dbb-9b2b20218538