This makes the ArrayBuiltinsAssembler consistent with the StringBuiltinsAssembler
and paves the way for tools that expect the assemblers to have a common structure.

Change-Id: I7470fc2cf144f9cc2fdbcee99b31daed267550be
Reviewed-on: https://chromium-review.googlesource.com/889933Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50920}