HTML Imports: Let dynamically-added imports sync.

Dynamically added import was marked as async. But it was unspec-ed behavior
and should be sync as usual imports.

TEST=import-dynamic-async-notblock-sync.html,import-dynamic-block-async.html, improt-dynamic-block-sync.html, import-dynamic-descendant-block.html
BUG=367351

Review URL: https://codereview.chromium.org/265453003

git-svn-id: svn://svn.chromium.org/blink/trunk@173149 bbb929c8-8fbe-4397-9dbb-9b2b20218538