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Add a semi-universal way to handle enum type conversions in MetaData.
In this CL, two enum properties from ImageViewBase were converted to
demo the usage.

BUG=938501,957272

Change-Id: I7e9df7fc7ec10bfff1303f1b0f537cab4d5068d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600858
Commit-Queue: Wei Li <weili@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658780}