diagnostics_service: add default case for DiagnosticRoutineEnum

This default case ensures that CrosHealthd can add additional routines
without modifying the DiagnosticsService. Routines will have to be
explicitly added to the conversion list if they want to be used.

Bug: b:158658869
Change-Id: I9a12db071e59b71e12bb6ce91a6b116c1d9ed52c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363315
Commit-Queue: Trent Begin <tbegin@chromium.org>
Auto-Submit: Trent Begin <tbegin@chromium.org>
Reviewed-by: Paul Moy <pmoy@chromium.org>
Reviewed-by: Oleh Lamzin <lamzin@google.com>
Reviewed-by: Mahmoud Gawad <mgawad@google.com>
Cr-Commit-Position: refs/heads/master@{#799684}

chromeos/components/telemetry_extension_ui/diagnostics_service_converters.cc

@@ -122,9 +122,9 @@ base::Optional<health::mojom::DiagnosticRoutineEnum> Convert(
       return health::mojom::DiagnosticRoutineEnum::kPrimeSearch;
     case cros_healthd::mojom::DiagnosticRoutineEnum::kBatteryDischarge:
       return health::mojom::DiagnosticRoutineEnum::kBatteryDischarge;
+    default:
+      return base::nullopt;
   }
-  NOTREACHED();
-  return base::nullopt;
 }
 
 std::string Convert(mojo::ScopedHandle handle) {