Remove abstraction which is no longer necessary

Use AssignedNodes() directly. The abstraction here is no longer
necessary since we removed IncrementalShadowDOM flag.

Bug: 887827
Change-Id: Id5c7e4139ad89ce2ade6efd62bdfc1cbb50c0340
Reviewed-on: https://chromium-review.googlesource.com/1249467
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594976}
2 files changed