The motivation of this change was originally to preserve is_growable
flag over PostMessage in d8. Adding a more general constructor that
uses SharedArrayBuffer::Contents.

Change-Id: Ib8f6c36d659e91f6cfb6487f56de34fa7e8841a9
Bug: v8:8564
Reviewed-on: https://chromium-review.googlesource.com/c/1383093
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59184}