PartitionAlloc: move seedPage and similar items to a global variable.

(This is a part of re-landing the reviewed but reverted
https://codereview.chromium.org/105503009/
in incremental pieces.)

TBR=tsepez@chromium.org
BUG=327095

Review URL: https://codereview.chromium.org/127323002

git-svn-id: svn://svn.chromium.org/blink/trunk@164654 bbb929c8-8fbe-4397-9dbb-9b2b20218538