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Polymer2 does not support HTML element extensions, so all such elements need to
be converted to wrappers.

Bug: 818279
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I1cb05f71bff45f04e5ca5f5e64489813d8db7136
Reviewed-on: https://chromium-review.googlesource.com/972332Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544912}