PartitionAlloc: Apply a temporary bandaid to fix a Mac performance regression.

Coding for a more comprehensive way is proceeding. Short-term, this will help
confirm the analysis.

BUG=330798

Review URL: https://codereview.chromium.org/127433003

git-svn-id: svn://svn.chromium.org/blink/trunk@164671 bbb929c8-8fbe-4397-9dbb-9b2b20218538