[heap] No leakage of objects-visiting.h outside of heap.

[heap] No leakage of objects-visiting.h outside of heap.

This prevents the internal objects-visiting.h to be usable outisde of
the "heap" directory. The static object visitation is only usefull
within the GC and is now properly encapsulated.

R=hpayer@chromium.org,mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1328003002

Cr-Commit-Position: refs/heads/master@{#30716}