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The underlying dropdown menu model already supports showing icons and
secondary labels.

This CL add such support to the combobox model. When an item is selected
the main label is shown inside in the combobox together with the icon.
Secondary labels are displayed only inside the dropdown menu.

Bug: 1044038
Change-Id: Ibe3cc37464bfd8e416aeadec62827b8ca103d13f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207386
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770807}