Free an empty partitionAlloc partition page even if it is the last page.

This causes us to use less memory in the case where an uncommon bucket size is
briefly used for a single allocation.

(Reviewed at https://codereview.chromium.org/26108003 but landing in a new
branch due to git issues with branch-off-a-branch)

R=tsepez@chromium.org
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/27153002

git-svn-id: svn://svn.chromium.org/blink/trunk@159605 bbb929c8-8fbe-4397-9dbb-9b2b20218538