Expose the steps for incremental marking and idle scavenge more directly in
NewSpace. Adjust the NewSpace and Heap interfaces to allow callers to be more
clear about how they are interacting with inline allocation steps. This refactor
prepares the ground for more consumers of inline allocation steps (e.g. sampling
heap profiler.)

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1404523002

Cr-Commit-Position: refs/heads/master@{#31814}