The name {CallerOwnedBuffer} does not make sense in all situations,
especially if such an object is returned instead of being passed as
argument.
I am working on moving the wasm wire bytes off the JS heap, and hence
will return unowned references via the API. To prepare this change, I
deprecate the existing {CallerOwnedBuffer} and introduce a new
{BufferReference} struct with proper field names.

R=titzer@chromium.org, adamk@chromium.org

Bug: v8:7868
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ic8953951447038a831b15a336a52a199bfbeafd5
Reviewed-on: https://chromium-review.googlesource.com/1108207Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53929}