Move autofocus handling out of post attach callback.

autofocus attribute was a source of a lot of seucrity issues because it
dispatched an event during Document::updateLayout(). We make autofocus
handing asynchronous.

We need to update a lot of tests.
 - We use waitUntilLoadedAndAutofocused() for tests in which using autofocus is
   essential.
 - Otherwise, we just call Element::focus(), or use waitUntilLoadedAndAutofocused().

BUG=324707

Review URL: https://codereview.chromium.org/138743004

git-svn-id: svn://svn.chromium.org/blink/trunk@165275 bbb929c8-8fbe-4397-9dbb-9b2b20218538