Leverage build_webui() in c/b/r/settings/chromeos.

build_webui() has several benefits over the previous custom build
pipeline.

 1) It is the standardized WebUI build pipeline.
 2) Abstracts away a lot of low-level logic, by removing direct calls
    to the underlying WebUI GN rules (preprocess_if_expr,
    html_to_wrapper, css_to_wrapper, ts_library etc).
 3) Minifies CSS files and <style> tags within HTML files. Specifically
    the size of the 3 bundled files is reduced as follows (in bytes)
     - lazy_load.rollup.js 1083615 -> 1039424
     - os_settings.rollup.js 1415498 -> 1374929
     - shared.rollup.js 863754 -> 851644
 4) Makes it possible to get code coverage metrics for CrOS Settings.

Bug: 1431477
Change-Id: I48de9da73a9dda4f760f19a340cb6b918c380ba0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4548690


Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147403}