This allows very large arrays being joined to incrementally,
on-demand allocate the internal buffer. Previously, join
would allocate the buffer upfront and all at once. Large,
sparse arrays will use less memory.

Bug: chromium:897404
Change-Id: Id914b14a7c55a62834f63ad602bdb45363249075
Reviewed-on: https://chromium-review.googlesource.com/c/1303538
Commit-Queue: Peter Wong <peter.wm.wong@gmail.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57075}