Add functionality to window.history.go test page.

- Added back/forward buttons.
- Added pushState() button.  This is necessary because if the pushState
  call is executed by injected JavaScript, then the navigation is
  registered as a client redirect.
- Display placeholder state object string when a popstate event with a
  null state object is received.

BUG=673022

Review-Url: https://codereview.chromium.org/2583733002
Cr-Commit-Position: refs/heads/master@{#439020}

ios/testing/data/http_server_files/history_go.html

@@ -27,10 +27,16 @@ found in the LICENSE file. -->
     onclick="goNoParameter()" /><br>
 <input type="button" value="go-zero" id="go-zero"
     onclick="goZero()" /><br>
+<input type="button" value="go-back" id="go-back"
+    onclick="goBack()" /><br>
+<input type="button" value="go-forward" id="go-forward"
+    onclick="goForward()" /><br>
 <input type="button" value="go-2" id="go-2"
     onclick="go2()" /><br>
 <input type="button" value="go-back-2" id="go-back-2"
     onclick="goBack2()" /><br>
+<input type="button" value="push-state-with-hash" id="push-state-with-hash"
+    onclick="pushStateWithHash()" /><br>
 
 </body>
 </html>

ios/testing/data/http_server_files/history_go.js

@@ -12,7 +12,8 @@ window.onload = function() {
 // Populates pop-state-received-div and state-object-div upon a popstate event.
 var onPopstate = function(e) {
   updatePopStateReceivedText(true);
-  updateStateObjectText(e.state);
+  var stateText = e.state ? e.state : '(NO STATE OBJECT)';
+  updateStateObjectText(stateText);
 };
 
 // Populates hash-change-received-div upon receiving of a hashchange event.
@@ -64,6 +65,16 @@ var goZero = function() {
   window.history.go(0);
 }
 
+var goBack = function() {
+  onButtonTapped();
+  window.history.back();
+}
+
+var goForward = function() {
+  onButtonTapped();
+  window.history.forward();
+}
+
 var go2 = function() {
   onButtonTapped();
   window.history.go(2);
@@ -73,3 +84,8 @@ var goBack2 = function() {
   onButtonTapped();
   window.history.go(-2);
 }
+
+var pushStateWithHash = function() {
+  onButtonTapped();
+  window.history.pushState('STATE_OBJECT', 'Title', '#hash');
+}