This CL completely rearchitects the ownership of array buffer backing stores,
consolidating ownership into a {BackingStore} C++ object that is tracked
throughout V8 using unique_ptr and shared_ptr where appropriate.

Overall, lifetime management is simpler and more explicit. The numerous
ways that array buffers were initialized have been streamlined to one
Attach() method on JSArrayBuffer. The array buffer tracker in the
GC implementation now manages std::shared_ptr<BackingStore> pointers,
and the construction and destruction of the BackingStore object itself
handles the underlying page or embedder-allocated memory.

The embedder API remains unchanged for now. We use the
v8::ArrayBuffer::Contents struct to hide an additional shared_ptr to
keep the backing store alive properly, even in the case of aliases
from live heap objects. Thus the embedder has a lower chance of making
a mistake. Long-term, we should move the embedder to a model where they
manage backing stores using shared_ptr to an opaque backing store object.

R=mlippautz@chromium.org
BUG=v8:9380,v8:9221

Change-Id: I48fae5ac85dcf6172a83f252439e77e7c1a16ccd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1584323
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62572}