Bug 1262368 - Fix browser_UITour_heartbeat.js to work on e10s. r=MattN

MozReview-Commit-ID: 6DtCPx9wa1r

browser/components/uitour/test/browser.ini

@@ -31,7 +31,6 @@ skip-if = os == "linux" # Linux: Bug 986760, Bug 989101.
 [browser_UITour_detach_tab.js]
 [browser_UITour_forceReaderMode.js]
 [browser_UITour_heartbeat.js]
-skip-if = e10s # Bug 1240747 - UITour.jsm not e10s friendly.
 [browser_UITour_loop.js]
 skip-if = true # Bug 1225832 - New Loop architecture is not compatible with test.
 [browser_UITour_loop_panel.js]

browser/components/uitour/test/head.js

@@ -288,6 +288,9 @@ function loadUITourTestPage(callback, host = "https://example.org/") {
                 callbackMap.set(index, arg);
                 fnIndices.push(index);
                 let handler = function(msg) {
+                  // Please note that this handler assumes that the callback is used only once.
+                  // That means that a single gContentAPI.observer() call can't be used to observe
+                  // multiple events.
                   browser.messageManager.removeMessageListener(proxyFunctionName + index, handler);
                   callbackMap.get(index).apply(null, msg.data);
                 };