function onGotPicture(rect, picture)
        {
            if (!rect || !picture || !this._target) {
                callback(null, null);
                return;
            }
            WebInspector.PaintProfilerSnapshot.load(this._target, picture, callback.bind(null, rect));
        }
@param {?Array.<number>} rect
@param {?string} picture
@this {WebInspector.LayerPaintEvent}