HTMLLinkElement should not run script inside ::insertedInto

This was done in r165624 but was wrong, you cannot ever synchronously
dispatch the load or error event from inside didFinish since it means
you run can end up running script inside ::insertedInto.

It's not clear from the bug what the issue was, but the timing in the
test that I updated was also wrong since it had <link> elements firing
their load events synchronously while parsing.

There might be a real timing bug in imports, but firing the event
synchronously inside ::didFinish() isn't how to fix it.

BUG=336113, 357435, 357445

Review URL: https://codereview.chromium.org/214513011

git-svn-id: svn://svn.chromium.org/blink/trunk@170268 bbb929c8-8fbe-4397-9dbb-9b2b20218538