[DevTools] Fix console error when reloading DevTools frontend.

Source/devtools/front_end/devtools.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(window) {
 
 // DevToolsAPI ----------------------------------------------------------------
 
 /**
  * @constructor
@@ skipping 806 matching lines @@
     styleElement.textContent = "html /deep/ * { min-width: 0; min-height: 0; }";
     window.document.head.appendChild(styleElement);
 }
 
 function windowLoaded()
 {
     window.removeEventListener("DOMContentLoaded", windowLoaded, false);
     installBackwardsCompatibility();
 }
 
-if (window.document.readyState === "complete" || window.document.readyState === "interactive")
+if (window.document.head && (window.document.readyState === "complete" || window.document.readyState === "interactive"))
     installBackwardsCompatibility();
 else
     window.addEventListener("DOMContentLoaded", windowLoaded, false);
 
 // UITests ------------------------------------------------------------------
 
 if (window.domAutomationController) {
     var uiTests = {};
 
     uiTests._tryRun = function()
@@ skipping 14 matching lines @@
     uiTests.testSuiteReady = function(testSuiteConstructor, testBase)
     {
         uiTests._testSuite = testSuiteConstructor(window.domAutomationController);
         uiTests._tryRun();
     };
 
     window.uiTests = uiTests;
 }
 
 })(window);