Implement BackgroundFetchManager.fetch() and struct traits

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

Index: third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom
diff --git a/third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom b/third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom
index 9092281d0e4a846e2171735d1dc7850af980e3be..35c42c31f17b2f1b9d059a26a1c341d40a9f128d 100644
--- a/third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom
+++ b/third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom
@@ -15,15 +15,27 @@ struct IconDefinition {
   string type;
 };
 
+struct BackgroundFetchOptions {
+  array<IconDefinition> icons;
+  int64 total_download_size;
+  string title;
+};
+
 struct BackgroundFetchRegistration {
   string tag;
   array<IconDefinition> icons;
-  int64 total_download_size = 0;
-  string title = "";
+  int64 total_download_size;
+  string title;
 };
 
 interface BackgroundFetchService {
-  // TODO(peter): Implement support for the `fetch()` function in Mojo.
+  // Creates a new Background Fetch registration identified by |tag| with the
+  // given |options| for the sequence of |requests|.
+  Fetch(int64 service_worker_registration_id,
+        string tag,
+        BackgroundFetchOptions options)
+      => (BackgroundFetchError error,
+          BackgroundFetchRegistration? registration);
 
   // Updates the user interface for the Background Fetch identified by the
   // |service_worker_registration_id| and the |tag|.