Currently this function requires the caller to hold a mutex for the
large page chunk hashtable and performs a hashtable lookup.

This patch adds a header sentinel field in each MemoryChunk. The field
is then used to distinguish large object slots from ordinary slots.

Bug: chromium:915233
Change-Id: I9fbeeb4f07f49573d0a21f9a2cc934370e417d68
Reviewed-on: https://chromium-review.googlesource.com/c/1391752
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58732}