GitLab

As the second step of implementing "Imperative Shadow DOM Distribution API",
this CL make assign function to assign nodes to slot in manual slotting mode.

This CL also introduces HeapVector for each slot
to store the nodes that assign introduced.

BUG=869308

Change-Id: I9b02a70f601364123fb565221d3def760408bc2a
Reviewed-on: https://chromium-review.googlesource.com/1166925Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kyoko Muto <kymuto@google.com>
Cr-Commit-Position: refs/heads/master@{#583178}