[heap] refactor inline allocation step code

[heap] refactor inline allocation step code

Once I improved byte accounting done for incremental mark in [1], there is some
code duplication that becomes apparent. This commit refactors the duplicated
code into a private method on NewSpace. This also makes it easy to add new
consumers of inline allocation steps in the future.

[1] https://codereview.chromium.org/1274453002/

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1351983002

Cr-Commit-Position: refs/heads/master@{#30921}