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In the process, removed flattenhtml=true to stop inlining
about_webapks.css, and serving it from its own URL instead.

Also updating invalidations_ui.cc to use webui::AddResourcePathsBulk()
which was forgotten in a previous similar CL.

Bug: 1132403
Change-Id: I4b399d344bdb684e8f75e1261cdb4f64f7fb94ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612096
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840842}