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Also change GRIT configuration from flattenhtml to preprocess where possible.

Bug: 866236
Change-Id: Ief42c461cc6b61d2a3719e823c261053d2bf205d
Reviewed-on: https://chromium-review.googlesource.com/c/1383395
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618273}