DOMUI Prefs: Add delete/backspace handling to deletable item lists

chrome/browser/resources/options/options_page.css

 body {
   -webkit-user-select: none;
   cursor: default;
   font-size: 100%;
 }
 
 #settings-title {
   -webkit-padding-end: 24px;
   color: #53637d;
   font-size: 200%;
@@ skipping 419 matching lines @@
   -webkit-transition: .15s opacity;
   background-color: transparent;
   /* TODO(stuartmorgan): Replace with real images once they are available. */
   background-image: url("../../../app/theme/close_bar.png");
   border: none;
   height: 16px;
   opacity: 1;
   width: 16px;
 }
 
-list > *:not(:hover):not([editing]) .close-button,
+list > *:not(:hover):not([lead]) .close-button,
 list[disabled] .close-button,
 list .close-button[disabled] {
   opacity: 0;
   pointer-events: none;
 }
 
 list .close-button:hover {
   background-image: url("../../../app/theme/close_bar_h.png");
 }
 
@@ skipping 93 matching lines @@
 }
 
 .suboption {
   -webkit-margin-start: 16px;
 }
 
 html[os=mac] label > input[type=checkbox],
 html[os=mac] label > input[type=radio] {
   margin-top: 2px;
 }