Bug 1504133 - Make pageloader tests wait for TalosPowers to become available before starting. r=aswan

Depends on D11261

Differential Revision: https://phabricator.services.mozilla.com/D11262

--HG--
extra : moz-landing-system : lando

testing/talos/talos/pageloader/api.js

@@ -39,6 +39,7 @@
 
 ChromeUtils.import("resource://gre/modules/Services.jsm");
 ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");
+ChromeUtils.import("resource://gre/modules/Timer.jsm");
 
 XPCOMUtils.defineLazyServiceGetter(this, "aomStartup",
                                    "@mozilla.org/addons/addon-manager-startup;1",
@@ -93,9 +94,25 @@ this.pageloader = class extends ExtensionAPI {
     ]);
 
     if (env.exists("MOZ_USE_PAGELOADER")) {
-      // This is async but we're delibeately not await-ing or return-ing
+      // TalosPowers is a separate WebExtension that may or may not already have
+      // finished loading. tryLoad is used to wait for TalosPowers to be around
+      // before continuing.
+      async function tryLoad() {
+        try {
+          ChromeUtils.import("resource://talos-powers/TalosParentProfiler.jsm");
+        } catch (err) {
+          await new Promise(resolve => setTimeout(resolve, 500));
+          return tryLoad();
+        }
+
+        return null;
+      }
+
+      // talosStart is async but we're deliberately not await-ing or return-ing
       // it here since it doesn't block extension startup.
-      talosStart();
+      tryLoad().then(() => {
+        talosStart();
+      });
     }
   }