For property cells with cell type kConstantType, we can compute an
appropriate JavaScript type based on the current value of that cell.
Numbers cannot use Type::Of here, because the type might be too precise,
so we handle smi and heap number specially.

R=jarin@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1400483002

Cr-Commit-Position: refs/heads/master@{#31171}