PA: Add PA_UNLIKELY to allocation failure handlers

It should be very rare that allocation failure handlers (new handlers)
are called and another allocation attempt is performed. So annotates
them with PA_UNLIKELY.

Change-Id: Idc39d68b3d84f1d46f47dffbd676c19b53cd48d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5691002
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1325976}