Oilpan: Fix thinko in assert in Heap.h

Thanks to Finnur for pointing this out.
R=ager@chromium.org, finnur@chromium.org
BUG=

Review URL: https://codereview.chromium.org/314643002

git-svn-id: svn://svn.chromium.org/blink/trunk@175349 bbb929c8-8fbe-4397-9dbb-9b2b20218538