HTML Imports: Take care of cycles according to the spec.

This fixes for regression that blocks import cycle indefinitely.
The change has two points:

 * Let isBlockingFollowers() see HTMLImportChild::isFirst(). The
   isFirst() corresponds to the notion of "marked as branch" in the spec.
 * Make CustomElementMicrosaskImportStep aware of tree shuffling done by
   HTMLImportChild::normalize().

TEST=import-cycle-*.html
R=dglazkov@chromium.org, dominicc@chromium.org
BUG=373461

Review URL: https://codereview.chromium.org/289993004

git-svn-id: svn://svn.chromium.org/blink/trunk@174141 bbb929c8-8fbe-4397-9dbb-9b2b20218538