Factor out IterableToList into a helper stub to save space. There are
two callers now, TypedArrayFrom and ConstructByIterable, and it is
~2.5kb so we save space by doing this.

Increase test coverage to cover more of the branching in CSA.

This is doesn't follow the control flow in the spec exactly - see the
big code comment for an explanation.

Change-Id: Ief39e93c4202cb7bf0e28a39dc6aa81b8b9c59d2
Reviewed-on: https://chromium-review.googlesource.com/908755
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51377}