Remove extra allocation observer.
Minor tweaks.

Change-Id: Ic7e6e2f8cb39ff960960cca6cc3ece46438a4bd5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1588405Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61130}