REGRESSION(r171966): Custom elements in async imports don't get upgrade.

For these custom elements, microstask steps are queued but never get dispatched
as the import isn't part of microtask dispatch tree.

This change supresses microtask queue creation for async imports so that
the microtask steps from these imports go to the top-level queue.

TEST=import-custom-element-async-resolve.html
R=dominicc@chromium.org, dglazkov@chromium.org
BUG=365956

Review URL: https://codereview.chromium.org/249563003

git-svn-id: svn://svn.chromium.org/blink/trunk@173402 bbb929c8-8fbe-4397-9dbb-9b2b20218538