Tidy up some ASSERTs in PartitionAlloc.h

Make them clearer by not calling the same function lots of times.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/126383002

git-svn-id: svn://svn.chromium.org/blink/trunk@164619 bbb929c8-8fbe-4397-9dbb-9b2b20218538