Oilpan: Clear out memory when the memory is added to a free list

This CL changes a place where memory is cleared out.
Before this CL, memory was cleared out in Heap::allocate().
After this CL, memory is cleared out when added to a free list.

This is good for:

- preventing freed memory from being exploited.
- improving performance of Heap::allocate().

BUG=

Review URL: https://codereview.chromium.org/353343002

git-svn-id: svn://svn.chromium.org/blink/trunk@177304 bbb929c8-8fbe-4397-9dbb-9b2b20218538