DOMUI Prefs: Change subpage close button to an actual button element

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 21 matching lines @@
   top: 0;
   bottom: 0;
   z-index: 10;
   padding: 20px;
   display: -webkit-box;
   -webkit-box-align: center;
   -webkit-box-pack: center;
 }
 
 .close-subpage {
+  background-color: transparent;
   /* TODO(stuartmorgan): Replace with real images once they are available. */
   background-image: url("../../../app/theme/close_bar.png");
+  border: none;
   position: relative;
   top: 12px;
   height: 16px;
   width: 16px;
 }
 
 .close-subpage:hover {
   background-image: url("../../../app/theme/close_bar_h.png");
 }
 
@@ skipping 412 matching lines @@
 label.radio > span,
 .suboption {
   -webkit-margin-start: 16px;
   display: block;
 }
 
 html[os=mac] label.checkbox > input,
 html[os=mac] label.radio > input {
   margin-top: 2px;
 }