This adds an {AccountingAllocator} to the {WasmEngine}, separate from
the allocator used per Isolate. It is in preparation of being able to
share engines across multiple Isolates. For now we just add up the
stats from both allocators until a public API for the engine becomes
available.

R=ahaas@chromium.org
TEST=cctest/test-wasm-shared-engine/SharedEngineRunImported
BUG=v8:7424

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ia915a1ae4aa7ebed27073b7b6bd067e31717a6ea
Reviewed-on: https://chromium-review.googlesource.com/1127788
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54316}