Also turn off autorunning micro tasks on workers

instead, we rely on v8recursionscope to run microtasks on workers as well. Before, the microtask queue was managed in blink an only existed on the
main thread, but now that it's managed by v8 per isolate, it's safe to use.

BUG=374798
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/298673002

git-svn-id: svn://svn.chromium.org/blink/trunk@174452 bbb929c8-8fbe-4397-9dbb-9b2b20218538