Storage manager: Hide "Offline files" when Google Drive volume is not mounted.

chrome/browser/resources/options/chromeos/storage_manager.html

 <div id="storageManagerPage" class="page" hidden>
   <div class="close-button"></div>
   <h1 i18n-content="storageManagerPage"></h1>
 
   <div id="storage-manager-message-low-space"
        class="storage-manager-message-area">
     <div class="storage-manager-message-title">
       <img src="space_low.svg"></img>
       <span i18n-content="storageLowMessageTitle"></span>
     </div>
@@ skipping 26 matching lines @@
               class="storage-manager-item-size"></span>
       </div>
       <div class="storage-manager-subitem">
         <a is="action-link"
            id="storage-manager-label-downloads"
            i18n-content="storageSubitemLabelDownloads"></a>
         <span id="storage-manager-size-downloads"
               class="storage-manager-item-size"
               i18n-content="storageSizeCalculating"></span>
       </div>
-      <div class="storage-manager-subitem">
+      <div id="storage-manager-item-drive-cache"
+           class="storage-manager-subitem" hidden>
         <a is="action-link"
            id="storage-manager-label-drive-cache"
            i18n-content="storageSubitemLabelDriveCache"></a>
         <span id="storage-manager-size-drive-cache"
               class="storage-manager-item-size"
               i18n-content="storageSizeCalculating"></span>
       </div>
       <div class="storage-manager-subitem">
         <a is="action-link"
            id="storage-manager-label-browsing-data"
@@ skipping 30 matching lines @@
   </div>
 
   <div class="action-area">
     <div class="button-strip">
       <button id="storage-confirm" class="default-button"
           i18n-content="done">
       </button>
     </div>
   </div>
 </div>