PartitionAlloc: simplify freeing logic for full pages.

If the page becomes empty in the "page was full" path, simply bounce the page
back through the free slow path. This unifies logic for empty pages and makes
sure we don't miss anything by two different code paths for a similar situation.

BUG=327095
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/133903008

git-svn-id: svn://svn.chromium.org/blink/trunk@164999 bbb929c8-8fbe-4397-9dbb-9b2b20218538