HTML Imports: Fix race conditions on HTMLImportsChild

There are two race conditions in HTMLImportChild.

1. ensureLoader() is called in stateDidChange(), which is triggered via a timer.
   That means there is a timing in which m_customElementMicrotaskStep isn't created but
   accompanying <link rel=import> is in tree. This allows some microtask can slip away
   from the blocking import.

2. HTMLImport::isDone() can return true when m_customElementMicrotaskStep is YET to be
   created, that can happen the call on ensureLoader().

This change covers these two cases. It also fixes a trivial flakiness in
import-custom-element-dup-resolve.html

TEST=import-custom-element-dup-resolve.html
R=dglazkov@chromium.org, dominicc@chromium.org
BUG=371654

Review URL: https://codereview.chromium.org/272243004

git-svn-id: svn://svn.chromium.org/blink/trunk@173794 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Disable repaint-after-layout

There has been enough bugs reported that we shouldn't be
pushing for it for M36. As we expect to fix the bugs by
M37, keeping the flag on to avoid rebaseline churn.

BUG=371856

Review URL: https://codereview.chromium.org/271173004

git-svn-id: svn://svn.chromium.org/blink/trunk@173795 bbb929c8-8fbe-4397-9dbb-9b2b20218538