Performing lookups on the prototype chain in the stub avoids a
bunch of slow-path runtime calls. For now, only receivers with
dictionary-mode properties do this; fast-mode receivers will follow
if it's beneficial.

(previously landed as r42751 / 82e10f5f)

Review-Url: https://codereview.chromium.org/2652213003
Cr-Commit-Position: refs/heads/master@{#42785}