chromeos: Introduce SetClient() on ash::mojom::SystemTray interface

chrome/browser/chrome_content_browser_manifest_overlay.json

 {
   "name": "content_browser",
   "display_name": "Chrome",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         "renderer": [
           "autofill::mojom::AutofillDriver",
           "autofill::mojom::PasswordManagerDriver",
           "extensions::StashService",
           "metrics::mojom::LeakDetector",
           "rappor::mojom::RapporRecorder",
           "startup_metric_utils::mojom::StartupMetricHost",
           "translate::mojom::ContentTranslateDriver"
         ],
         "gpu": [
           "metrics::mojom::CallStackProfileCollector"
         ],
         "ash": [
           // Under classic ash the browser provides some of the ash interfaces
           // to itself.
           "ash::mojom::LocaleNotificationController",
           "ash::mojom::NewWindowClient",
           "ash::mojom::ShelfController",
           "ash::mojom::ShutdownController",
           "ash::mojom::SystemTray",
-          "ash::mojom::SystemTrayClient",
           "ash::mojom::WallpaperController",
           "ash::mojom::WallpaperManager",
           "ash::mojom::VolumeController",
           "ash::mojom::VpnList",
           "keyboard::mojom::Keyboard"
         ],
         "mash:launchable": [
           "mash::mojom::Launchable"
         ]
       },
@@ skipping 24 matching lines @@
           // TODO(beng): These should be moved to a separate capability.
           "mojom::OmniboxPageHandler",
           "mojom::PluginsPageHandler",
           "mojom::SiteEngagementUIHandler",
           "mojom::UsbInternalsPageHandler"
         ]
       }
     }
   }
 }