MD History: Simplify flex layout in <history-list>

chrome/browser/resources/md_history/grouped_list.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-collapse/iron-collapse.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://history/constants.html">
 <link rel="import" href="chrome://history/history_item.html">
 <link rel="import" href="chrome://history/shared_style.html">
 
 <dom-module id="history-grouped-list">
   <template>
     <style include="shared-style">
       :host {
+        display: block;
         overflow: auto;
         position: relative;
-        padding: var(--first-card-padding-top) var(--card-padding-side);
       }
 
       #main-container {
         align-items: center;
         display: flex;
         flex-direction: column;
+        padding: var(--first-card-padding-top) var(--card-padding-side);
       }
 
       .domain-heading {
         align-items: center;
         display: flex;
         height: 40px;
         padding: 0 20px;
       }
 
       .domain-count {
@@ skipping 74 matching lines @@
                 </template>
               </iron-collapse>
             </div>
           </template>
         </div>
       </template>
     </div>
     </template>
   <script src="chrome://history/grouped_list.js"></script>
 </dom-module>