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

Bug: 1132403
Change-Id: Ief759ba6c33303d2d955b853ffa44de3de467642
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611328
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840528}