This adds an additional V8 API to get the backing store of an array
buffer. Unlike the existing API, the backing store comes wrapped
in a std::shared_ptr, making lifetime management with the embedder
explicit. This obviates the need for the old GetContents() and
Externalize() APIs, which will be deprecated in a future CL.

Contributed by titzer@chromium.org


Bug: v8:9380
Change-Id: I8a87f5dc141dab684693fe536b636e33f6e45173
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1807354Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63883}