Replacing it with SMI_ACCESSORS.
This change makes accesses to Smi fields in objects more regular (the
accessors now always consume/return an int rather than a Smi*), which
avoids a bunch of manual Smi::FromInt() and Smi::value() conversions,
and is a step on the way towards being able to generate objects-inl.h.

Review URL: https://codereview.chromium.org/1371893002

Cr-Commit-Position: refs/heads/master@{#30975}