Oilpan: Clarify ownership of split-off heaps.

- BaseHeap::split() should return PassOwnPtr<BaseHeap>.
- BaseHeap::merge() should take a PassOwnPtr<BaseHeap> argument.

BUG=420515
TEST=None; No behavior changes.

Review URL: https://codereview.chromium.org/669323002

git-svn-id: svn://svn.chromium.org/blink/trunk@184255 bbb929c8-8fbe-4397-9dbb-9b2b20218538