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Use rollup along with existing Polymer CSS build and Uglify tools
to bundle Polymer 3 pages in optimized builds.

This will be used by the extensions Web UI in a followup CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1817293

Bug: 965770
Change-Id: Id4538b5628d1d59824300598082d89ea316eb628
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846752
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709327}