[heap] No leakage of gc-idle-time-handler.h outside of heap.

[heap] No leakage of gc-idle-time-handler.h outside of heap.

This prevents the internal gc-idle-time-handler.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=ulan@chromium.org

Review URL: https://codereview.chromium.org/1368983002

Cr-Commit-Position: refs/heads/master@{#30939}