Manual revert of r159875 and r159833 in PartitionAllocTest.

Subsequent to r160111, we no longer need the foul hacks introduced into the
test. This is a manual revert because other changes have moved the code along;
and also because I kept the new RELEASE_ASSERTs added to PageAllocator.cpp,
which are useful.

TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/34173002

git-svn-id: svn://svn.chromium.org/blink/trunk@160135 bbb929c8-8fbe-4397-9dbb-9b2b20218538