Oilpan: Support ephemerons - really weak maps where the strong value does not keep the weak key alive.

R=ager@chromium.org, haraken@chromium.org, wibling@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/340773002

git-svn-id: svn://svn.chromium.org/blink/trunk@176601 bbb929c8-8fbe-4397-9dbb-9b2b20218538