HTML Import: Implement @async attribute

The plumbing has been there, that is isCreatedByParser.
This change renamed it to "Sync" and check the @async attribute
to decide if it is sync or not.

TEST=import-async.html
BUG=352311
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/196573012

git-svn-id: svn://svn.chromium.org/blink/trunk@169169 bbb929c8-8fbe-4397-9dbb-9b2b20218538