Fix context menu missing box-shadow for Runo on Chrome (#7966)

Change-Id: I318761527a695c741f52b4c18e9f8ea943f89017

themes/src/main/themes/VAADIN/themes/runo/common/common.scss

@@ -84,7 +84,7 @@
 	margin: 1px 0 4px 0;
 }
 .v-contextmenu {
-	background: #e9eced url(../tabsheet/img/tab-bg.png);
+	background: #e9eced url(../tabsheet/img/tab-bg.png) content-box;
 	font-family: "Trebuchet MS", geneva, helvetica, arial, tahoma, verdana, sans-serif;
 	background-color: #f6f7f7;
 	color: #464f52;