Instead of {base::AtomicNumber<intptr_t>} use {std::atomic<size_t>},
since we really want to store a size_t in there, and only abused
negative values before to avoid a compare-and-swap loop.

R=mstarzinger@chromium.org

Bug: v8:7570
Change-Id: Ibff0fe0550396f11b343f7e3c098ccf94f6e8dbb
Reviewed-on: https://chromium-review.googlesource.com/1049067Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53098}