Add mojo service to get notified of vsync.

mojo/services/vsync/interfaces/vsync.mojom

Index: mojo/services/vsync/interfaces/vsync.mojom
diff --git a/mojo/services/vsync/interfaces/vsync.mojom b/mojo/services/vsync/interfaces/vsync.mojom
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index 0000000000000000000000000000000000000000..5cc66a1a34483eebfe20ee06f5d5354b033947e1
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+++ b/mojo/services/vsync/interfaces/vsync.mojom
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+// Copyright 2015 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.
+
+[DartPackage="mojo_services"]
+module vsync;
+
+interface VSyncProvider {
+  // Waits for the next vsync and returns its timestamp once it happens. The
+  // timestamps can only be compared with other calls of this method.
+  // Only one callback can be parked at a given time.
+  AwaitVSync() => (int64 time_stamp);
+};