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This CL makes null and undefined distinguishable for dictionary
members whose type is a nullable interface.

It changes Blink-side expectation about dict->hasFoo(), if
a dictionary member `foo` is in the case. Before this CL,
dict->hasFoo() means dict->foo() does not return null pointers,
but after this CL, dict->foo() can returns nullptr.


Bug: 855968
Change-Id: I4dfddf0ae79703a69294aa627cff86f5e7c59252
Reviewed-on: https://chromium-review.googlesource.com/1113176Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571009}