Allow posting a Task to Microtasks

...not just a static callback. This will be used for
<img> loading and potential other future users.

R=rafaelw@chromium.org
BUG=

Review URL: https://codereview.chromium.org/242123005

git-svn-id: svn://svn.chromium.org/blink/trunk@171965 bbb929c8-8fbe-4397-9dbb-9b2b20218538