Add FontFaceSource interface and make it available on Document

third_party/WebKit/Source/core/css/FontFaceSource.idl

Index: third_party/WebKit/Source/core/css/FontFaceSource.idl
diff --git a/third_party/WebKit/Source/core/css/FontFaceSource.idl b/third_party/WebKit/Source/core/css/FontFaceSource.idl
new file mode 100644
index 0000000000000000000000000000000000000000..c7263bf01784cf993a11b7a6d20d85a531aec972
--- /dev/null
+++ b/third_party/WebKit/Source/core/css/FontFaceSource.idl
@@ -0,0 +1,12 @@
+// 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.
+
+// spec: https://drafts.csswg.org/css-font-loading/#font-face-source
+
+[
+  LegacyTreatAsPartialInterface,
+  NoInterfaceObject
+] interface FontFaceSource {
+  [MeasureAs=DocumentFonts] readonly attribute FontFaceSet fonts;
+};