service worker: Add console.warn output to enable -expected.txt files

third_party/WebKit/LayoutTests/http/tests/serviceworker/chromium/navigation-preload-origin-trial-methods.html

Index: third_party/WebKit/LayoutTests/http/tests/serviceworker/chromium/navigation-preload-origin-trial-methods.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/serviceworker/chromium/navigation-preload-origin-trial-methods.html b/third_party/WebKit/LayoutTests/http/tests/serviceworker/chromium/navigation-preload-origin-trial-methods.html
index cd493da2c3fb12d5bd1febd136fcabda6ea79513..2fc1faada3d8bb56bb51c033c5f73505082194ef 100644
--- a/third_party/WebKit/LayoutTests/http/tests/serviceworker/chromium/navigation-preload-origin-trial-methods.html
+++ b/third_party/WebKit/LayoutTests/http/tests/serviceworker/chromium/navigation-preload-origin-trial-methods.html
@@ -10,6 +10,9 @@ generate_token.py  http://127.0.0.1:8000 ServiceWorkerNavigationPreload -expire-
 <script src="../resources/test-helpers.js"></script>
 <script src="./resources/get_interface_names.js"></script>
 <script>
+console.warn('This test relies on console message comparison so there can ' +
+    'be different -expected.txt files for virtual test suites.');
+
 function check_methods(t, script, scope) {
   var registration;
   var worker;