The backing store fixed array for collections needs to be allocated
in LOS if it exceeds the maximum regular heap object size.

Drive-by-fix: Only store fixed array map once as per TODO.

Bug: chromium:784862
Change-Id: I6b4dd2e45153ae107171e21bc7448e0d9b54b0ed
Reviewed-on: https://chromium-review.googlesource.com/771150Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49378}