WebUI: Add support for resource_path_rewrites to generate_grd

Support more general resource path rewrites by adding the option to
pass such rewrites from the caller in the form:
"original_file_path|resource_path_for_grd"
Leverage this to rename rollup files, and remove the special-case logic
that was used for this purpose originally.

Bug: 1132403
Change-Id: Icffa7d85497ab000044fed1ebcd9542cb5b869bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476623
Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817961}