Before we can set the length of the created array in CSA, first check
that it's possible and will do what we want. I.e. check
a) that the length is writable
b) the backing store is not copy-on-write and
c) the old length is not greater than the new length (as otherwise later
insertion past the end could restore values from the original
constructor).

If not then fall back on Runtime::kSetProperty.

Bug: chromium:804177
Change-Id: Id0e452f9d160704bbd71e87a075ba4e3983729a7
Reviewed-on: https://chromium-review.googlesource.com/880922
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50818}