Rename FrameUtil.isReady to .isDocumentReady

Better differentiation from .isLoaded

src/annotator/frame-observer.js

@@ -38,7 +38,7 @@ class FrameObserver {
 
   _addFrame (frame) {
     if (FrameUtil.isAccessible(frame)) {
-      FrameUtil.isReady(frame, () => {
+      FrameUtil.isDocumentReady(frame, () => {
         frame.contentWindow.addEventListener('unload', () => {
           this._removeFrame(frame);
         });

src/annotator/util/frame-util.js

@@ -43,7 +43,7 @@ function isValid (iframe) {
   return iframe.className !== 'h-sidebar-iframe';
 }
 
-function isReady (iframe, callback) {
+function isDocumentReady (iframe, callback) {
   if (iframe.contentDocument.readyState === 'loading') {
     iframe.contentDocument.addEventListener('DOMContentLoaded', function () {
       callback();
@@ -70,5 +70,5 @@ module.exports = {
   isAccessible: isAccessible,
   isValid: isValid,
   isLoaded: isLoaded,
-  isReady: isReady,
+  isDocumentReady: isDocumentReady,
 };