instance_class_name takes up space unnecessarily, and %_ClassOf and
class_name implement [[Class]] which isn't part of ES2015+ anymore.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I3a73f732ad83a616817fde9992f4e4d584638fa8
Reviewed-on: https://chromium-review.googlesource.com/776683Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51309}