Add helper to FrameUtil that calls back when an iframe is ready

src/annotator/plugin/cross-frame.coffee

@@ -90,10 +90,7 @@ module.exports = class CrossFrame extends Plugin
 
     _handleFrame = (frame) ->
       if !FrameUtil.isAccessible(frame) then return
-      if frame.contentDocument.readyState != 'complete'
-        frame.addEventListener 'load', ->
-          _injectToFrame(frame)
-      else
+      FrameUtil.isLoaded frame, () ->
         _injectToFrame(frame)
 
     _iframeUnloaded = (frame) ->

src/annotator/util/frame-util.js

@@ -43,10 +43,21 @@ function isValid (iframe) {
   return iframe.className !== 'h-sidebar-iframe';
 }
 
+function isLoaded (iframe, callback) {
+  if (iframe.contentDocument.readyState !== 'complete') {
+    iframe.addEventListener('load', function () {
+      callback();
+    });
+  } else {
+    callback();
+  }
+}
+
 module.exports = {
   findFrames: findFrames,
   hasHypothesis: hasHypothesis,
   injectHypothesis: injectHypothesis,
   isAccessible: isAccessible,
   isValid: isValid,
+  isLoaded: isLoaded,
 };