Verify that appendChild(script) does not perform a microtask checkpoint

This potential bug was raised in the review for the previous patch:
https://codereview.chromium.org/130983011/

BUG=340322

Review URL: https://codereview.chromium.org/146043008

git-svn-id: svn://svn.chromium.org/blink/trunk@166962 bbb929c8-8fbe-4397-9dbb-9b2b20218538