function renderTabIndex(oButton, oRm){
		if (oButton._bExcludeFromTabChain) {
			oRm.attr("tabindex", -1);
		}
	}
Renders tabindex with value of "-1" if required by  <code>_bExcludeFromTabChain</code> property.
@param {sap.m.Button} oButton The sap.m.Button to be rendered
@param {sap.ui.core.RenderManager} oRm The RenderManager that can be used for writing to the Render-Output-Buffer