Remove flattenhtml from Print Preview

With the update to GM2, locations in Print Preview that were using
inlined images have now been updated to iron-icons and .cr-icons, so
flattenhtml can be removed entirely in print_preview_resources.grd, and
replaced with preprocess in print_preview_resources_vulcanized.grd.

This requires modifying tools/grit/grit/node/include.py, to allow the
use of preprocess in <include>s.

Bug: None
Change-Id: If2b099b47ef136ff8487ea1cc812ce8f29d02036
Reviewed-on: https://chromium-review.googlesource.com/c/1377500
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616943}
5 files changed