Revert of Revert of Perform a microtask checkpoint for document.write('<script><\/script>') (https://codereview.chromium.org/153913010/)

Reason for revert:
Wrong revert



Original issue's description:
> Revert of Perform a microtask checkpoint for document.write('<script><\/script>') (https://codereview.chromium.org/130983011/)
> 
> Reason for revert:
> Broke MutationObserver consistency.
> 
> 
> Original issue's description:
> > 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
> > 
> > Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=166445
> 
> TBR=eseidel@chromium.org,esprehn@chromium.org,adamk@chromium.org,rafaelw@chromium.org,philipj@opera.com
> NOTREECHECKS=true
> NOTRY=true
> BUG=340322
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=166960

TBR=eseidel@chromium.org,esprehn@chromium.org,adamk@chromium.org,rafaelw@chromium.org,philipj@opera.com
NOTREECHECKS=true
NOTRY=true
BUG=340322

Review URL: https://codereview.chromium.org/160293002

git-svn-id: svn://svn.chromium.org/blink/trunk@166961 bbb929c8-8fbe-4397-9dbb-9b2b20218538