Only Microtask::performCheckpoint() if recursionLevel is 0

This maintains the invariant that microtask work (e.g. Mutation Observer callbacks) always occur on an empty page script stack.

Previously, it was possible for a microtask to fire before the outer script had exited on document.write('<script>') or showModalDialog, etc..

R=adamk@chromium.org, adamk
BUG=352181

Review URL: https://codereview.chromium.org/184043002

git-svn-id: svn://svn.chromium.org/blink/trunk@169770 bbb929c8-8fbe-4397-9dbb-9b2b20218538