This property might be useful for fast '===' check.

R=dgozman@chromium.org,yangguo@chromium.org

Bug: none
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;luci.chromium.try:linux_chromium_rel_ng;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iabc3555ce1ec2c14cf0ccd40b7d964ae144e7352
Reviewed-on: https://chromium-review.googlesource.com/1226411Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56095}