Support choosing multiple files with fileSystem.chooseEntry.

chrome/test/data/extensions/api_test/file_system/open_multiple_existing/manifest.json

 {
-  "name": "chrome.fileSystem get writable file entry",
+  "name": "chrome.fileSystem open existing files",
   "version": "0.1",
-  "description": "Test for chrome.fileSystem.getWritableFileEntry",
+  "description":
+      "Test for chrome.fileSystem.chooseFile opening multiple existing files.",
   "app": {
     "background": {
       "scripts": ["background.js"]
     }
   },
   "permissions": [
     {
       "fileSystem": []
     }
   ]
 }