CrosDBusService: Move private test code into anonymous namespace

- In particular the MockProxyResolutionService class. If any other
  unit test file adds a type with the same name this creates a
  ODR violation.
- This results in objects of the second type silently being treated
  as if it was this type which can cause difficult to debug
  "seemingly impossible" behavior especially with virtual function
  calls
- Typically the test code could also go in the anonymous namespace
  except in this case external code is friending this class so it
  needs to remain visible
- Some further reading here [1] and style guide [2]

[1] - https://akrzemi1.wordpress.com/2016/11/28/the-one-definition-rule/
[2] - https://google.github.io/styleguide/cppguide.html#Internal_Linkage

Bug: 1276194
Test: chromeos_unittests
Change-Id: I954173a34ac4b092c82945428ef9c25d8f97debd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3314058
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#947796}
diff --git a/chromeos/dbus/services/cros_dbus_service_unittest.cc b/chromeos/dbus/services/cros_dbus_service_unittest.cc
index be332a7..260c016 100644
--- a/chromeos/dbus/services/cros_dbus_service_unittest.cc
+++ b/chromeos/dbus/services/cros_dbus_service_unittest.cc
@@ -27,6 +27,8 @@
 
 namespace chromeos {
 
+namespace {
+
 class MockProxyResolutionService
     : public CrosDBusService::ServiceProviderInterface {
  public:
@@ -34,6 +36,8 @@
                            exported_object));
 };
 
+}  // namespace
+
 class CrosDBusServiceTest : public testing::Test {
  public:
   CrosDBusServiceTest() = default;