Previously, this was just a field on the WasmResult, which is not
allowed according to the style guide.
A special r-value accessor for the value is needed for the cases where
the contained type is not copyable, e.g. unique_ptr.

R=titzer@chromium.org

Bug: v8:8238
Change-Id: Ia3c14c4c62c3c2e07f1dc4594f1bc9d1da88f91e
Reviewed-on: https://chromium-review.googlesource.com/c/1290974
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56823}