[DevTools] Fix null tooltip errors

third_party/WebKit/Source/devtools/front_end/ui/Tooltip.js

 // Copyright (c) 2015 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.
 /**
  * @unrestricted
  */
 UI.Tooltip = class {
   /**
    * @param {!Document} doc
    */
@@ skipping 12 matching lines @@
 
   /**
    * @param {!Document} doc
    */
   static installHandler(doc) {
     new UI.Tooltip(doc);
   }
 
   /**
    * @param {!Element} element
-   * @param {!Element|string} tooltipContent
+   * @param {?Element|string} tooltipContent
    * @param {string=} actionId
    * @param {!Object=} options
    */
   static install(element, tooltipContent, actionId, options) {
-    if (typeof tooltipContent === 'string' && tooltipContent === '') {
+    if (!tooltipContent) {
       delete element[UI.Tooltip._symbol];
       return;
     }
     element[UI.Tooltip._symbol] = {content: tooltipContent, actionId: actionId, options: options || {}};
   }
 
   /**
    * @param {!Element} element
    */
   static addNativeOverrideContainer(element) {
@@ skipping 142 matching lines @@
   },
 
   /**
    * @param {!Element|string} x
    * @this {!Element}
    */
   set: function(x) {
     UI.Tooltip.install(this, x);
   }
 });