[Media Router] Merge MediaRouterDesktop and MediaRouterMojoImpl.

These two classes serve the same purpose: MediaRouter implementation
for desktop platforms.  This is a straightforward merge of the two
classes and should result in no functional changes.

As part of the merge, declarations in the .h were grouped into a more
logical order, and definitions in the .cc were re-ordered to match the
declaration order.

A small refactor was done to allow unit tests to avoid initializing
Media Route Providers.

Bug: 1299101
Change-Id: I43f142fd4b03947e3245944c52102d5725103849
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4743803
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: Mark Foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1185300}

chrome/browser/media/router/BUILD.gn

@@ -123,8 +123,6 @@ static_library("router") {
       "mojo/media_router_debugger_impl.h",
       "mojo/media_router_desktop.cc",
       "mojo/media_router_desktop.h",
-      "mojo/media_router_mojo_impl.cc",
-      "mojo/media_router_mojo_impl.h",
       "mojo/media_router_mojo_metrics.cc",
       "mojo/media_router_mojo_metrics.h",
       "mojo/media_sink_service_status.cc",
@@ -305,7 +303,7 @@ source_set("unittests") {
       "logger_list_unittest.cc",
       "media_router_feature_unittest.cc",
       "mojo/media_router_debugger_impl_unittest.cc",
-      "mojo/media_router_mojo_impl_unittest.cc",
+      "mojo/media_router_desktop_unittest.cc",
       "mojo/media_sink_service_status_unittest.cc",
       "providers/cast/app_activity_unittest.cc",
       "providers/cast/cast_activity_manager_unittest.cc",