Fixes the MaybeObject->Object conversion in ObjectStats to allow Smis,
rather than just HeapObjects.

Change-Id: I845613c47bb6ca696d444a025100b471fb385980
Reviewed-on: https://chromium-review.googlesource.com/1049925Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53070}