DevTools: Add JSDoc for static methods, fix JSDoc types and induced errors

Source/devtools/front_end/components/TimelineGrid.js

 /*
  * Copyright (C) 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2008, 2009 Anthony Ricaud <rik@webkit.org>
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
@@ skipping 173 matching lines @@
         if (!dividerOffsets) {
             var dividersData = WebInspector.TimelineGrid.calculateDividerOffsets(calculator, this._dividersElement.clientWidth);
             dividerOffsets = dividersData.offsets;
             precision = dividersData.precision;
             printDeltas = false;
         }
 
         var dividersElementClientWidth = this._dividersElement.clientWidth;
 
         // Reuse divider elements and labels.
-        var divider = this._dividersElement.firstChild;
-        var dividerLabelBar = this._dividersLabelBarElement.firstChild;
+        var divider = /** @type {?Element} */ (this._dividersElement.firstChild);
+        var dividerLabelBar = /** @type {?Element} */ (this._dividersLabelBarElement.firstChild);
 
         const minWidthForTitle = 60;
         var lastPosition = 0;
         var lastTime = 0;
         for (var i = 0; i < dividerOffsets.length; ++i) {
             if (!divider) {
                 divider = document.createElement("div");
                 divider.className = "resources-divider";
                 this._dividersElement.appendChild(divider);
 
@@ skipping 138 matching lines @@
 
     /** @return {number} */
     zeroTime: function() { },
 
     /** @return {number} */
     maximumBoundary: function() { },
 
     /** @return {number} */
     boundarySpan: function() { }
 }