Enhancements to PartitionAlloc to support threading and arbitrary allocation sizes.

BUG=267094
TEST=WTF_PartitionAlloc.GenericAlloc
R=abarth@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/21666003

git-svn-id: svn://svn.chromium.org/blink/trunk@155544 bbb929c8-8fbe-4397-9dbb-9b2b20218538