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Add a new Polymer element, source-select, to display scan sources (e.g.
flatbed, document feeder, etc.) in a dropdown. Styling will be added in
subsequent CLs.

Bug: 1059779
Change-Id: I88603298c3c749905527b5d74216f6285d984d51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424988Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Commit-Queue: Jesse Schettler <jschettler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811407}