Web Inspector: Exception thrown when hovering network waterfall graphs during reload

Web Inspector: Exception thrown when hovering network waterfall graphs during reload
https://bugs.webkit.org/show_bug.cgi?id=162850
<rdar://problem/28579653>

Reviewed by Brian Burg.

* UserInterface/Views/ResourceTimelineDataGridNode.js:
(WebInspector.ResourceTimelineDataGridNode.prototype._mouseoverRecordBar):
Check that the node's data grid reference is valid before use.
(WebInspector.ResourceTimelineDataGridNode):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@206722 268f45cc-cd09-0410-ab3c-d52691b4dbfc