PDF Viewer: Use HTML imports polyfill

- Imports the HTML imports polyfill into the PDF viewer, and adjusts
as needed to allow the polyfill to finish loading before creating
Polymer elements that require imports.
- Reverts
https://chromium-review.googlesource.com/c/chromium/src/+/1405955,
which created an exception for the PDF viewer to use HTML imports after
they are deprecated.
- Testing: PDF viewer + Print Preview with and browser_tests with filter
*PDFExtension* with the --disable-blink-features=HTMLImports.

Bug: 924031
Change-Id: Ife1c2402933f3eec399927212c6738955cdb8b0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1481607
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641277}
3 files changed