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Previously these were defined at the global scope, which does not play well with
the tools used for migrating to Polymer3. It also pollutes the global namespace.

Bug: 1026426
Change-Id: I8e99c6f83e78a8ac8e1a4073102f60abe70c6448
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2002002
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732215}