Simplify superpage freeing in partition shutdown code.

Superpage freeing can be made simpler now that we explicitly track superpages.
We no longer have to infer them.

I also removed the dynamic allocation (courtesy of Vector) in the shutdown
path. Aside from being bad form, this change is required for another patch I
have in-flight which changes Vector to be backed by partitionAlloc.

BUG=270531
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/25690002

git-svn-id: svn://svn.chromium.org/blink/trunk@158736 bbb929c8-8fbe-4397-9dbb-9b2b20218538