Oilpan: Revert prompt freeing of collection backings.

Promptly freeing collection backings leads to performance
regressions on the perf bots. Reverting the related CLs:

r180987: "Oilpan: Don't free collection backings that have been allocated on a different thread."
r180967: "Oilpan: Coalesce free-list entries when the number of promptly freed objects exceeds its bound."
r180957: "Oilpan: Promptly free collection backings when possible."

TBR=ager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/515863002

git-svn-id: svn://svn.chromium.org/blink/trunk@181027 bbb929c8-8fbe-4397-9dbb-9b2b20218538