Check the receiver_map for the dictionary mode bit instead of
comparing the properties map against the HashTableMap.

Bug: 
Change-Id: Iebf3118f00fd0afc8f7f13e88f373282c099f682
Reviewed-on: https://chromium-review.googlesource.com/578324
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46775}