HTML Imports: Let root state recalculation delay

It has a race which hits an ASSERT().
This change makes it delayed to run it in its own call stack.

BUG=390176
TEST=import-insert-domcontnetloaded.html
R=dglazkov@chromium.org, dominicc@chromium.org

Review URL: https://codereview.chromium.org/368743002

git-svn-id: svn://svn.chromium.org/blink/trunk@177853 bbb929c8-8fbe-4397-9dbb-9b2b20218538