Move more sources from modules.gypi to subdirectories.

third_party/WebKit/Source/modules/background_sync/BUILD.gn

Index: third_party/WebKit/Source/modules/background_sync/BUILD.gn
diff --git a/third_party/WebKit/Source/modules/background_sync/BUILD.gn b/third_party/WebKit/Source/modules/background_sync/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..da346d842b575a2810c263db4418fd75038afc07
--- /dev/null
+++ b/third_party/WebKit/Source/modules/background_sync/BUILD.gn
@@ -0,0 +1,21 @@
+# Copyright 2016 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.
+
+import("//third_party/WebKit/Source/modules/modules.gni")
+
+blink_modules_sources("background_sync") {
+  sources = [
+    "ServiceWorkerGlobalScopeSync.h",
+    "ServiceWorkerRegistrationSync.cpp",
+    "ServiceWorkerRegistrationSync.h",
+    "SyncCallbacks.cpp",
+    "SyncCallbacks.h",
+    "SyncError.cpp",
+    "SyncError.h",
+    "SyncEvent.cpp",
+    "SyncEvent.h",
+    "SyncManager.cpp",
+    "SyncManager.h",
+  ]
+}