This should be the fastest case, as we can just copy the backing store
directly. Adding this test so that we can monitor if upcoming changes
regress this path.

BUG=v8:5977

Change-Id: I021a199061ac845f265a906bda68b7ad3e8d5708
Reviewed-on: https://chromium-review.googlesource.com/461183Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44225}

BUG=v8:5977

Change-Id: Ic756fd44a945f98d51c0914dcc6c3b82111d170d
Reviewed-on: https://chromium-review.googlesource.com/456419Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43978}