Assert the V8 is correctly configured to not autorun microtasks

BUG=none
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/291823002

git-svn-id: svn://svn.chromium.org/blink/trunk@174541 bbb929c8-8fbe-4397-9dbb-9b2b20218538