Added role attribute to UI-Grid-Header.html

Element path:
#\31 559651691891-left-grid-container > .ui-grid-header.ng-scope[role="rowgroup"] > .ui-grid-top-panel > .ui-grid-header-viewport > .ui-grid-header-canvas > .ui-grid-header-cell-wrapper > .ui-grid-header-cell-row[role="row"]

Environment
Chrome version 74.0.3729.169 This accessibility issue was found using Accessibility Insights for Web 2.4.1 (axe-core 3.2.2), a tool that helps find and fix accessibility issues. Get more information & download this tool at http://aka.ms/AccessibilityInsights.

Required ARIA children role not present: cell columnheader rowheader gridcell

Author: MsDev88

packages/core/src/templates/ui-grid/ui-grid-header.html

@@ -14,6 +14,7 @@
             role="row"
             class="ui-grid-header-cell-row">
             <div
+              role="columnheader"
               class="ui-grid-header-cell ui-grid-clearfix"
               ng-repeat="col in colContainer.renderedColumns track by col.uid"
               ui-grid-header-cell