Bug 1692086 - Position and scale submenu arrows correctly to spec. r=dao

I also noticed while trying out this patch that the context menu navigation
icons really need to also scale with the font size, so I've fixed that here
along with the submenu arrows.

Differential Revision: https://phabricator.services.mozilla.com/D107570

browser/themes/shared/contextmenu.inc.css

@@ -13,8 +13,8 @@
 }
 
 #context-navigation > .menuitem-iconic > .menu-iconic-left > .menu-iconic-icon {
-  width: 16px;
-  height: 16px;
+  width: 1.25em;
+  height: auto;
   margin: 7px;
   -moz-context-properties: fill;
   fill: currentColor;

toolkit/themes/windows/global/menu.css

@@ -150,6 +150,11 @@ menucaption {
     list-style-image: url("chrome://global/skin/icons/menu-arrow.svg");
     -moz-context-properties: fill;
     fill: currentColor;
+    margin-inline-end: 1.5em;
+  }
+
+  .menu-right > image {
+    width: 1em;
   }
 
   .menu-right:-moz-locale-dir(rtl) {