COW arrays were previously handled in the C++ pre-processing runtime
function.  The Torque version forgot a "EnsureWritableFastElements".
This CL fixes that.

Bug: chromium:967254
Change-Id: Ifbf89e57cfe724e61316b8abc226f7e8a262fce2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1630675Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61835}