This CL introduces Tagged_t and AtomicTagged_t typedefs which represent
the storage type of tagged values in V8 heap.

Bug: v8:7703
Change-Id: Ib57e85ea073eaf896b6406cf0f62adcef9a114ce
Reviewed-on: https://chromium-review.googlesource.com/c/1352294Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57878}