WebUI: Use css_to_wrapper() for shared cr_elements/ Polymer styles.

Also passing mwb_shared_icons.ts to html_to_wrapper() instead of
html_to_js(), per latest approach for Polymer icons files.

Bug: 1322682
Change-Id: I855e00fd6f0b56d8ca6b996bd9b3343e1f0698d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3636008
Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1001680}