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- Use Polymer.Templatize instead of extending Templatizer behavior,
  since Templatizer behavior is now legacy/deprecated.
- Add a closure compiler target for settings idle load

This is in preparation for migrating settings idle load to Polymer 3

Bug: 1042558
Change-Id: I820f1a51aff09478277f3aae1ad1611fdc334a59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023472
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735746}