base/allocator clean-ups

No functional change.

Change-Id: Icec05521d1e037200a48bca8b5a61dbaddb72099
Reviewed-on: https://chromium-review.googlesource.com/c/1394733
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620991}