Add support for refreshing contents of providers which don't support watchers.

chrome/common/extensions/docs/templates/intros/fileSystemProvider.html

 <h2 id="manifest">Manifest</h2>
 <p>You must declare the "fileSystemProvider" permission and section
 in the <a href="manifest">extension manifest</a>
 to use the File System Provider API.
 For example:</p>
 <pre data-filename="manifest.json">
 {
   "name": "My {{platform}}",
   ...
   "permissions": [
     "fileSystemProvider"
   ],
   ...
   "file_system_provider_capabilities": {
     "configurable": true,
+    "watchable": false,
     "multiple_mounts": true,
     <span id="manifest-source">"source": "network"</span>
   },
   ...
 }
 </pre>
 
 <p class="api_reference">
 The <code>file_system_provider</code> section must be declared as follows:
 {{+partials.manifest_type
    type:apis.extensions.manifestTypes.byName.FileSystemProviderCapabilities /}}
 </p>
 <p>
 Files app uses above information in order to render related UI elements
 approprietly. For example, if <code>configurable</code> is set to
 </code>true</code>, then a menu item for configuring volumes will be rendered.
 Similarly, if <code>multiple_mounts</code> is set to <code>true</code>, then
-Files app will allow to add more than one mount points from the UI.
+Files app will allow to add more than one mount points from the UI. If
+<code>watchable</code> is <code>false</code>, then a refresh button will be
+rendered. Note, that if possible you should add support for watchers, so
+changes on the file system can be reflected immediately and automatically.
 </p>
 
 <h2 id="overview">Overview</h2>
 <p>
 File System Provider API allows extensions to support virtual file systems,
 which are available in the file manager on Chrome OS.
 Use cases include decompressing archives and accessing files in a cloud
 service other than Drive.
 </p>
 
@@ skipping 68 matching lines @@
 extension is unloaded, or the extension calls the $(ref:unmount) method.
 </p>
 {{?is_apps}}
 <p>
 In case of acting as a file handler, the handled file may need to be stored
 to access it after either a reboot, or suspending and resuming an event page
 of the providing extension. In such case $(ref:fileSystem.retainEntry) and
 $(ref:fileSystem.restoreEntry) should be used.
 </p>
 {{/is_apps}}