[MD Bookmarks] Add hover border to sidebar splitter.

chrome/browser/resources/md_bookmarks/app.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/cr/ui/splitter.html">
 <link rel="import" href="chrome://bookmarks/api_listener.html">
 <link rel="import" href="chrome://bookmarks/constants.html">
 <link rel="import" href="chrome://bookmarks/dnd_manager.html">
 <link rel="import" href="chrome://bookmarks/list.html">
 <link rel="import" href="chrome://bookmarks/router.html">
 <link rel="import" href="chrome://bookmarks/shared_vars.html">
 <link rel="import" href="chrome://bookmarks/store.html">
 <link rel="import" href="chrome://bookmarks/toolbar.html">
@@ skipping 12 matching lines @@
       }
 
       #main-container {
         display: flex;
         flex-direction: row;
         flex-grow: 1;
         overflow: auto;
       }
 
       #splitter {
+        box-sizing: border-box;
         cursor: e-resize;
         flex: 0 0 var(--splitter-width);
+        opacity: 0;
+      }
+
+      #splitter:hover,
+      #splitter.splitter-active {
+        -webkit-border-start: 1px solid rgba(0, 0, 0, 0.1);
+        opacity: 1;
+        transition: opacity 100ms ease-out;
       }
 
       :host-context([dir='rtl']) #splitter {
         cursor: w-resize;
       }
 
       #sidebar {
         -webkit-padding-start: 3px;
         display: block;
         max-width: 40%;
@@ skipping 15 matching lines @@
        <div id="sidebar">
         <bookmarks-folder-node item-id="0" depth="-1"></bookmarks-folder-node>
       </div>
       <div id="splitter"></div>
       <bookmarks-list></bookmarks-list>
     </div>
     <bookmarks-router></bookmarks-router>
   </template>
   <script src="chrome://bookmarks/app.js"></script>
 </dom-module>