The GcSafe* methods rely on Heap internals and should thus belong to Heap.

Bug: 
Change-Id: I4e6468d51c4dda1d10e94568698e05bee1b56b40
Reviewed-on: https://chromium-review.googlesource.com/789935
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49630}