Reviewed by Timothy Hatcher.

        Web Inspector: speed up network panel rendering.
        https://bugs.webkit.org/show_bug.cgi?id=53397

        * inspector/front-end/DataGrid.js:
        (WebInspector.DataGrid.prototype.get scrollContainer):
        * inspector/front-end/NetworkPanel.js:
        (WebInspector.NetworkPanel.prototype.elementsToRestoreScrollPositionsFor):
        (WebInspector.NetworkPanel.prototype._positionSummaryBar):
        (WebInspector.NetworkPanel.prototype._createTable):
        (WebInspector.NetworkPanel.prototype._exportResource):
        (WebInspector.NetworkPanel.prototype._onScroll):
        * inspector/front-end/networkPanel.css:
        (.network-sidebar .data-grid.small tr.offscreen):
        (.network-sidebar .data-grid tr.offscreen):
        (.network-sidebar .data-grid tr.offscreen td):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@77127 268f45cc-cd09-0410-ab3c-d52691b4dbfc