The allocation observer step should be called only after the page flags
of the young large object are properly set up.

Bug: chromium:852420
Change-Id: I9f537a7c1d6b7aa15ccbe58006e3957a2ec2ab5c
Reviewed-on: https://chromium-review.googlesource.com/c/1477735Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59696}