Post a microtask to load <img> elements.

See the "stable state" (step 1) part of the algorithm at
http://www.whatwg.org/specs/web-apps/current-work/#the-img-element

This is a requirement for <picture>, because we need to wait with
loading until we know whether we're in a <picture> element.
It also makes it so that the order of setting .src and .crossorigin
does not matter.

Unfortunately, this means that some exceptions no longer have a
stack trace.

BUG=341047

Review URL: https://codereview.chromium.org/200923002

git-svn-id: svn://svn.chromium.org/blink/trunk@173943 bbb929c8-8fbe-4397-9dbb-9b2b20218538