Move ArgumentCodersCocoa.mm to WebKitPlatform. Needed for upcoming testing work.
<rdar://problem/117493028>
Pull request: https://github.com/WebKit/WebKit/pull/19562
Committed 269792@main (5bec434844a9): <https://commits.webkit.org/269792@main> Reviewed commits have been landed. Closing PR #19562 and removing active labels.