WebUI: Move web_ui_test.mojom and related code to CrOS-only folder.

These files are only used in legacy js2gtest tests, which are no longer
used on Desktop.

Note that chrome/test/base/ash/mojo_web_ui_browser_test.h which
consumes such bindings on the C++ side, is already limited to CrOS.

Bug: 1347740
Change-Id: I24baefaa539d0a69d2d3aacc7af48b81ee4f03e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5034588
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1226485}