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This allows ObserverList checking for ComboboxModel subclasses, which
have also been updated.

BUG=963982

Change-Id: Idc3b8a853261a1afd8f0c9132d34d1a681b10204
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614756Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660616}