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This change takes the drop-down-search-box from the "Add Cups Printer"
dialog and makes it a cr-element so that it can be used elsewhere.

- Updates cups_add_printers_dialog to use cr-searchable-drop-down

Bug: 866146
Test: browser_tests
Change-Id: Ib0d953c3dbb1f51dad94a73451eb924d12d1b39e
Reviewed-on: https://chromium-review.googlesource.com/1145774
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577722}