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Preparation for bundling the PDF Viewer's resources, step 2/3.

 - Add a pdf_viewer_wrapper.js top-level file, necessary to make tests
   work in both optimized_webui true/false modes
 - Update all PDF Viewer tests to import dependencies from the new
   file
 - Minor tweaks to keep Print Preview working, while using the same
   c/b/r/pdf/main.js file

Bug: 1163956
Change-Id: I92dde642f94edbcf08ff72c9cdb5c11bd9f7e8a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625334
Auto-Submit: dpapad <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843134}