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 - Options are sorted alphabetically (http://shortn/_nYkelby30A)
 - "Default" will be displayed before scanner capabilities are loaded
    or when the scanner does not report any sources.
 - A follow-up CL will update "getSourceTypeString_" to return
   localized strings.

Bug: 1059779
Test: browser_tests --gtest_filter=ScanningAppBrowserTest.All
Change-Id: I148e720ae3345baa069c385f6a977d8d90e809a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508366
Commit-Queue: Michael Checo <michaelcheco@google.com>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825406}