Another pair of {std::unique_ptr} and {size_t} that can be stored as
one {OwnedVector}, which allows to pass it as one thing.

R=mstarzinger@chromium.org

Bug: v8:7754
Change-Id: Ideac0dbd390ba8147b6620daa86f0d3da6c3b609
Reviewed-on: https://chromium-review.googlesource.com/1118236
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54091}