WebUI: Decouple polymer/html_to_js.py from polymer/polymer.py.

html_to_js() was reusing some parts of polymer_modulizer()'s
implementation. Given that polymer_modulizer() is no longer used and
will be deleted, need to decouple the tow GN rules to unblock the
deletion, by pulling out the parts that are needed by html_to_js().

Also adding dedicated tests for html_to_js.py, which did not exist
previously.

This is in preparation of fully remomving polymer_modulizer().

Bug: 1184053
Change-Id: I0c20577f215cf6009fa6f511ad9dfc3f61e4f41f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4173117
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1093655}