Ensure import details popup is hidden by default.

BUG=458510
TEST=browser_test: *FileManager*

Review URL: https://codereview.chromium.org/923023003

Cr-Commit-Position: refs/heads/master@{#316395}

ui/file_manager/file_manager/main.html

@@ -375,7 +375,7 @@
       </button>
     </div>
 
-    <div id="cloud-import-details" class="hidden">
+    <div id="cloud-import-details" class="hidden" hidden>
       <div class="banner">
         <div class="title" i18n-content="CLOUD_IMPORT_TITLE"></div>
         <div class="description" i18n-content="CLOUD_IMPORT_DESCRIPTION"></div>