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 - Modify Settings data source to serve generated files.
 - Add top-level JS/HTML files that will host the V3 version.
 - Use a JS import for the generated
   chrome://settings/search_settings.m.js as a proof of concept.
 - Update BUILD.gn files as needed.

Note that the dummy page is currently only available for
optimized_webui=false builds.

This is in preparation of migrating to Polymer 3.

Bug: 1026426
Change-Id: I53dc831bfad3cf4645fa91c643bd5e34d14190de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2024161
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736112}