Make WTF_PartitionAlloc.FreePageListPageTransitions reliable.

We need to setup a guaranteed contiguous super page span so that the allocation
of super page span metadata doesn't disturb what we're trying to test.

BUG=307705
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/27443002

git-svn-id: svn://svn.chromium.org/blink/trunk@159833 bbb929c8-8fbe-4397-9dbb-9b2b20218538