Move ResourceOwner on to the oilpan heap.

DocumentThreadableLoader isn't ready for oilpan, so drop its inheritance
of ResourceOwner.

This will better enable moving Resource to the oilpan heap, since
ResourceOwner can't easily support both oilpan and non-oilpan holders of
Resources if Resource is on the heap.

BUG=571190

Review URL: https://codereview.chromium.org/1569273004

Cr-Commit-Position: refs/heads/master@{#370214}
23 files changed