[heap] Limit friendship of the Heap class to essentials.

[heap] Limit friendship of the Heap class to essentials.

This makes it clear that only components within the "heap" directory
should be friends with the Heap class. The two notable exceptions are
Factory and Isolate which represent external interfaces into the heap.

R=mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1320843002

Cr-Commit-Position: refs/heads/master@{#30408}