Hook up BackgroundFetchServiceImpl::Fetch() to start a fetch

third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom

 // Copyright 2017 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.
 
 module blink.mojom;
 
 import "url/mojo/origin.mojom";
 
 enum BackgroundFetchError {
   NONE,
   DUPLICATED_TAG,
+  INVALID_ARGUMENT,
   INVALID_TAG
 };
 
 // Represents the definition of an icon developers can optionally provide with a
 // Background Fetch fetch. Analogous to the following structure in the spec:
 // https://wicg.github.io/background-fetch/#background-fetch-manager
 struct IconDefinition {
   string src;
   string sizes;
   string type;
@@ skipping 50 matching lines @@
                   string tag)
       => (BackgroundFetchError error,
           BackgroundFetchRegistration? registration);
 
   // Gets the sequence of tags for active Background Fetch registrations given
   // the |service_worker_registration_id|.
   GetTags(int64 service_worker_registration_id, url.mojom.Origin origin)
       => (BackgroundFetchError error,
           array<string> tags);
 };