Bug 984979 - Fix back button :active state on Windows 8, r=gijs

--HG--
extra : rebase_source : 39a2fa92fab331861e9503fb82a485435d56758a

browser/themes/windows/browser.css

@@ -892,6 +892,7 @@ toolbarbutton[sdk-button="true"][cui-areatype="toolbar"] > .toolbarbutton-icon {
   box-shadow: none !important;
 }
 
+#back-button:not([disabled="true"]):hover:active > .toolbarbutton-icon,
 #back-button[open="true"] > .toolbarbutton-icon {
   background-color: hsla(210,4%,10%,.12) !important;
   box-shadow: 0 1px 0 0 hsla(210,80%,20%,.1) inset !important;