Replace use of std containers with WTF's equivalents in html_slot_element.h

This CL replaces the use of std::array of std containers with WTF::Vector
and introduces LCSArray struct in html_slot_element.h.

Bug: 952716
Change-Id: I45d0dcbb63aed5198440bd7b94c5c693a05bd6fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1694801
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/master@{#675985}