Complete testing of the onDocumentReady function

I removed the `/* istanbul ignore test */` and created a facade to
emulate and iframe for testing.

src/annotator/test/frame-observer-test.js

@@ -4,7 +4,8 @@ import {
   onDocumentReady,
 } from '../frame-observer';
 
-describe('FrameObserver', () => {
+describe('annotator/frame-observer.js', () => {
+  describe('FrameObserver', () => {
     let container;
     let frameObserver;
     let onFrameAdded;
@@ -36,7 +37,11 @@ describe('FrameObserver', () => {
       document.body.appendChild(container);
       onFrameAdded = sandbox.stub();
       onFrameRemoved = sandbox.stub();
-    frameObserver = new FrameObserver(container, onFrameAdded, onFrameRemoved);
+      frameObserver = new FrameObserver(
+        container,
+        onFrameAdded,
+        onFrameRemoved
+      );
       sandbox.stub(console, 'warn');
     });
 
@@ -138,7 +143,52 @@ describe('FrameObserver', () => {
 
       await onDocumentReady(iframe);
       await waitForFrameObserver();
+
       assert.notCalled(onFrameAdded);
       assert.calledOnce(console.warn);
     });
+  });
+
+  describe('onDocumentReady', () => {
+    let fakeIFrame;
+    let fakeIFrameDocument;
+
+    beforeEach(() => {
+      fakeIFrameDocument = {
+        addEventListener: sinon.stub(),
+        readyState: 'loading',
+        location: {
+          href: 'about:blank',
+        },
+      };
+
+      fakeIFrame = {
+        contentWindow: {
+          document: fakeIFrameDocument,
+        },
+        addEventListener: sinon.stub(),
+        hasAttribute: sinon.stub(),
+      };
+    });
+
+    it('waits for the iframe load event to be triggered if the document is blank', () => {
+      fakeIFrameDocument.location.href = 'about:blank';
+      fakeIFrame.hasAttribute.returns(true);
+      onDocumentReady(fakeIFrame);
+
+      assert.calledWith(fakeIFrame.addEventListener, 'load');
+    });
+
+    it('waits for the iframe DOMContentLoaded event to be triggered if the document is loading', () => {
+      fakeIFrameDocument.location.href = 'about:srcdoc';
+      fakeIFrame.hasAttribute.returns(false);
+      fakeIFrameDocument.readyState = 'loading';
+      onDocumentReady(fakeIFrame);
+
+      assert.calledWith(
+        fakeIFrameDocument.addEventListener,
+        'DOMContentLoaded'
+      );
+    });
+  });
 });