Web Inspector: data grid resize is slow
https://bugs.webkit.org/show_bug.cgi?id=107291

Reviewed by Pavel Feldman.

- avoid (most of) unnecessary relayouts and style recalculation when dragging data grid resizer.

* inspector/front-end/DataGrid.js:
(WebInspector.DataGrid.prototype._positionResizers):
(WebInspector.DataGrid.prototype._resizerDragging):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140335 268f45cc-cd09-0410-ab3c-d52691b4dbfc