[heap] No leakage of incremental-marking.h outside of heap.

[heap] No leakage of incremental-marking.h outside of heap.

This prevents the internal incremental-marking.h to be usable outisde
of the "heap" directory. The logic inside that component is only useful
within the GC and is now properly encapsulated.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1374203002

Cr-Commit-Position: refs/heads/master@{#31010}