Make {First,Last}AssignedNode() be aware of Incremental Shadow DOM

Prevent a possible wrong usage of {First,Last}AssignedNode().

Bug: 776656
Change-Id: I5d84b062c92cd89e40e3c051ae08bcefea658d25
Reviewed-on: https://chromium-review.googlesource.com/896725
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533898}