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

chrome/browser/resources/md_history/synced_device_manager.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-item/paper-item.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu/paper-menu.html">
 <link rel="import" href="chrome://resources/html/cr/ui/position_util.html">
 <link rel="import" href="chrome://history/shared_style.html">
 <link rel="import" href="chrome://history/synced_device_card.html">
 
 <dom-module id="history-synced-device-manager">
   <style include="shared-style"></style>
   <template>
     <style>
       :host {
+        display: block;
         overflow: auto;
         padding-top: var(--first-card-padding-top);
       }
     </style>
     <template is="dom-repeat" items="[[syncedDevices_]]" as="syncedDevice">
       <history-synced-device-card device="[[syncedDevice.device]]"
           last-update-time="[[syncedDevice.lastUpdateTime]]"
           tabs="[[syncedDevice.tabs]]"
           separator-indexes="[[syncedDevice.separatorIndexes]]"
           searched-term="[[searchedTerm]]">
       </history-synced-device-card>
     </template>
   </template>
   <script src="chrome://history/synced_device_manager.js"></script>
 </dom-module>