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This prepares to have Combobox's model be used as a metadata and builder
property.

Bug: 938501
Change-Id: I5721408ab0059c2b6b54863b897005ef77e27bb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476514Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817709}