Simplify waitFor code (#92)

lib/FrameManager.js

@@ -178,25 +178,28 @@ class FrameManager extends EventEmitter {
    * @return {!Promise<undefined>}
    */
   async _waitForInFrame(selector, frame) {
-    let code = selector => {
+
-      let promise = new Promise(fulfill => {
+    function code(selector) {
+      if (document.querySelector(selector))
+        return Promise.resolve();
+
+      let callback;
+      const result = new Promise(fulfill => callback = fulfill);
+
+      const mo = new MutationObserver((mutations, observer) => {
         if (document.querySelector(selector)) {
-          fulfill();
+          observer.disconnect();
+          callback();
           return;
         }
-        new MutationObserver((mutations, observer) => {
-          if (document.querySelector(selector)) {
-            observer.disconnect();
-            fulfill();
-            return;
-          }
-        }).observe(document.documentElement, {
-          childList: true,
-          subtree: true
-        });
       });
-      return promise;
+      mo.observe(document.documentElement, {
-    };
+        childList: true,
+        subtree: true
+      });
+      return result;
+    }
+
     let contextId = undefined;
     if (!frame.isMainFrame()) {
       contextId = this._frameIdToExecutionContextId.get(frame._id);