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Specifically:
 - Update remaining usages of js/util.js to explicitly pull-in
   js/assert.js.
 - Update webui_resources.grd to not use flattenhtml=true for
   JS files.
 - Remove "// <include src=..." directives from affected files

This is in preparation of moving such files over to the
auto-generated webui_generated_resources.grd, which (purposefully)
no longer supports flattenhtml.

Bug: 1132403
Change-Id: Idb9e4df54348c3eac3cbe7152893e3d5c5bd82a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2503833
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823308}