[oilpan] Add missing header guard on Heap.h.

R=wibling@chromium.org
BUG=

Review URL: https://codereview.chromium.org/99623009

git-svn-id: svn://svn.chromium.org/blink/trunk@163521 bbb929c8-8fbe-4397-9dbb-9b2b20218538