[MD Bookmarks] Add draggable sidebar.

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/list.html">
 <link rel="import" href="chrome://bookmarks/shared_vars.html">
 <link rel="import" href="chrome://bookmarks/sidebar.html">
 <link rel="import" href="chrome://bookmarks/store.html">
 <link rel="import" href="chrome://bookmarks/toolbar.html">
 <link rel="import" href="chrome://bookmarks/util.html">
 
 <dom-module id="bookmarks-app">
   <template>
     <style>
       :host {
         display: flex;
         flex-direction: column;
         height: 100%;
         overflow: hidden;
       }
 
       #main-container {
         display: flex;
         flex-direction: row;
         flex-grow: 1;
-        overflow: auto;
+      }
+
+      #splitter {
+        border-left: 1px solid rgba(0, 0, 0, 0.08);
+        box-sizing: border-box;
+        cursor: e-resize;
+        flex: 0 0 var(--splitter-width);
       }
 
       bookmarks-sidebar {
-        flex: 0 0 var(--sidebar-width);
+        max-width: calc(100vw - 900px);
+        min-width: var(--min-sidebar-width);
+        width: var(--min-sidebar-width);
       }
 
       bookmarks-list {
         flex: 1;
       }
     </style>
-    <bookmarks-toolbar></bookmarks-toolbar>
+    <bookmarks-toolbar sidebar-width="[[sidebarWidth_]]"></bookmarks-toolbar>
     <div id="main-container">
-      <bookmarks-sidebar></bookmarks-sidebar>
+      <bookmarks-sidebar id="sidebar">

[tsergeant, 2017/03/14 03:38:51]

This can still live on one line.

[calamity, 2017/03/14 07:07:55]

Done.

+      </bookmarks-sidebar>
+      <div id="splitter"></div>
       <bookmarks-list></bookmarks-list>
     </div>
   </template>
   <script src="chrome://bookmarks/app.js"></script>
 </dom-module>