Oilpan: Clear the current allocation point after adding it to the free list.

TBR=ager@chromium.org
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/539043002

git-svn-id: svn://svn.chromium.org/blink/trunk@181367 bbb929c8-8fbe-4397-9dbb-9b2b20218538