Bug 1709591 - Use arrow-dropdown-16.svg for the Synced Tabs expander icon rather than rotating a right-pointing one. r=mhowell

Before this patch, we were showing a downward-pointing expander icon by taking a
rightward-pointing one and rotating it 90deg. That's fine, except for RTL, where we
tend to mirror rightward-pointing arrows with a `scale: -1 1;` rule. Because of
order of operations, the scale rule is applied first, and the 90deg rotation
that occurs afterwards ends up pointing the scaled arrow upwards.

Instead of special-casing the scale rule for this expander icon, I've gone ahead
and switched it to use arrow-dropdown-16.svg and gotten rid of the rotation. This
way, even if we mirror across the y-axis with `scale: -1 1`, the appearance will
not change.

Differential Revision: https://phabricator.services.mozilla.com/D115268

browser/themes/shared/customizableui/panelUI.inc.css

@@ -1620,7 +1620,7 @@ panelview .toolbarbutton-1,
 } /* END not Proton */
 
 .PanelUI-subView .subviewbutton-nav-down::after {
-  rotate: 90deg;
+  content: url(chrome://global/skin/icons/arrow-dropdown-16.svg);
 }
 
 .subviewbutton[shortcut]::after,