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

chrome/common/extensions/api/manifest_types.json

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // The type schemas for structured manifest items. Not actually a callable API.
 
 [
   {
     "namespace": "manifestTypes",
     "description": "Schemas for structured manifest entries",
@@ skipping 173 matching lines @@
           "configurable": {
             "type": "boolean",
             "optional": true,
             "description": "Whether configuring via <code>onConfigureRequested</code> is supported. By default: <code>false</code>."
           },
           "multiple_mounts": {
             "type": "boolean",
             "optional": true,
             "description": "Whether multiple (more than one) mounted file systems are supported. By default: <code>false</code>."
           },
+          "watchable": {
+            "type": "boolean",
+            "optional": true,
+            "description": "Whether setting watchers and notifying about changes is supported. By default: <code>false</code>."
+          },
           "source": {
             "$ref": "FileSystemProviderSource",
             "description": "Source of data for mounted file systems."
           }
         }
       }
     ]
   }
 ]