Have mash_browser_tests recreate BackgroundServiceManager per test

ui/views/mus/aura_init.h

Index: ui/views/mus/aura_init.h
diff --git a/ui/views/mus/aura_init.h b/ui/views/mus/aura_init.h
index fc37b5b691f8e64622edcfba22ac38c127550b90..e2937753d7af928289c9f213d86ad1976d97641e 100644
--- a/ui/views/mus/aura_init.h
+++ b/ui/views/mus/aura_init.h
@@ -66,7 +66,9 @@ class VIEWS_MUS_EXPORT AuraInit {
   MusClient* mus_client() { return mus_client_.get(); }
 
  private:
-  void InitializeResources(service_manager::Connector* connector);
+  // Attempts to initialize the resource bundle. Returns true if successful,
+  // otherwise false if resources cannot be loaded.
+  bool InitializeResources(service_manager::Connector* connector);
 
 #if defined(OS_LINUX)
   sk_sp<font_service::FontLoader> font_loader_;