This patch also fixes MaxReserved() to accound for page headers and
adds two tests for heap size and memory allocator size near OOM.

Bug: chromium:824214
Change-Id: I5bbe00a9d6a5798cdf4481861a10dca842244a63
Reviewed-on: https://chromium-review.googlesource.com/973614Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52184}