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- Shows a preview of a scanned document
- A follow-up CL will use and style this element

Bug: 1059779
Test: browser_tests --gtest_filter=ScanningUIBrowserTest.All
Change-Id: Ib5a9fe49f87c14059391900777247615cf7a6184
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500430
Commit-Queue: Michael Checo <michaelcheco@google.com>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821591}