Uncaught Exception in Web Inspector: TypeError: null is not an object (evaluating...

Uncaught Exception in Web Inspector: TypeError: null is not an object (evaluating 'dataGridNode.element.classList')
https://bugs.webkit.org/show_bug.cgi?id=151790

Reviewed by Timothy Hatcher.

* UserInterface/Views/DataGrid.js:
(WebInspector.DataGridNode):
(WebInspector.DataGridNode.prototype.get hidden):
(WebInspector.DataGridNode.prototype.set hidden):
Make hidden a property, so it can be set before the grid node's DOM element exists.
(WebInspector.DataGridNode.prototype.get selectable):
(WebInspector.DataGridNode.prototype.get element):
Add hidden style, if needed, when element is created.

* UserInterface/Views/TreeOutlineDataGridSynchronizer.js:
(WebInspector.TreeOutlineDataGridSynchronizer.prototype._treeElementVisibilityDidChange):
(WebInspector.TreeOutlineDataGridSynchronizer):
Don't access the grid node's element directly, since it may not exist yet.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@193375 268f45cc-cd09-0410-ab3c-d52691b4dbfc