This way we avoid the cyclic dependency between objects.h and heap.h and still
have one definition. Add a static assert that this size is indeed smaller than
the payload of a page.

Follow ups can finally remove the dependency on spaces.h for all heap.h users.

R=ulan@chromium.org,bmeurer@chromium.org,vogelheim@chromium.og

Review-Url: https://codereview.chromium.org/2311203002
Cr-Commit-Position: refs/heads/master@{#39206}