Code Health: Rename/replace content::WebUI::RegisterMessageCallback().

Special thanks to thestig@chromium.org for writing the original
version of this patch at https://crrev.com/c/2496505.

This CL renames the original RegisterMessageCallback() as
RegisterDeprecatedMessageCallback() and adds a new version of
RegisterMessageCallback() that passes a base::Value::ConstListView in
the callback instead of const base::ListValue*, as base::ListValue is
now deprecated.

Most of the many callsites of the original RegisterMessageCallback()
are converted to use RegisterDeprecatedMessageCallback() for the
moment so that the migration to the new RegisterMessageCallback()
can be split into future CLs (with tracking bug
https://crbug.com/1243386).

We do, however, use the new API in a couple spots
(chrome/browser/ui/webui/crashes_ui.cc and
chrome/browser/ui/webui/settings/chromeos/ambient_mode_handler.cc)
to show that it works.

We also update the examples for RegisterMessageCallback in
docs/webui_explainer.md and docs/webui_in_components.md.

AX-Relnotes: N/A.
Bug: 1142540,1187062,1243386
Change-Id: I80e86815ad81a8f82bdc36960d0816e43efdf27b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3104691
Commit-Queue: Cammie Smith Barnes <cammie@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Owners-Override: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915292}