Oilpan: Another fix for the HashMapOfMembers test.

Conservative stack scanning is, ehm, conservative. If we check exact
heap sizes we cannot use conservative scanning as it can keep stuff
alive that should be dead.

This showed up when testing our ASAN build in release mode.

R=erik.corry@gmail.com
BUG=

Review URL: https://codereview.chromium.org/170433004

git-svn-id: svn://svn.chromium.org/blink/trunk@167345 bbb929c8-8fbe-4397-9dbb-9b2b20218538