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Add an option to make Web UI pages work with the HTMLImports polyfill in
optimized builds. This is in preparation for modifying Web UI pages to
work with the polyfill instead of relying on native imports.

Bug: 925517
Change-Id: I8f6ca8694bd73defa810c304f31b83dc5ec7d19c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566761
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653411}