Bug 1335778 - Synchronize navigate() for trigger methods using generators. r=ato

In the case when the trigger callback inside navigate() uses a generator,
the code has to be synchronized and needs to wait until the contained
command has been completed.

MozReview-Commit-ID: 8qKUMvH7HpS

--HG--
extra : rebase_source : 3bc63d130c370354dab27bf40bbf13ec441bd423

testing/marionette/listener.js

@@ -273,19 +273,22 @@ var loadListener = {
       this.start(command_id, timeout, startTime, true);
     }
 
-    try {
-      trigger();
-    } catch (e) {
+    return Task.spawn(function* () {
+      yield trigger();
+
+    }).then(val => {
+      if (!loadEventExpected) {
+        sendOk(command_id);
+      }
+
+    }).catch(err => {
       if (loadEventExpected) {
         this.stop();
       }
-      sendError(new UnknownCommandError(e.message), command_id);
-      return;
-    }
 
-    if (!loadEventExpected) {
-      sendOk(command_id);
-    }
+      sendError(err, command_id);
+      return;
+    });
   },
 }