Tidy up some ASSERTs in PartitionAlloc.h
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@164619 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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