Expose SignalSyncPoint through MGL api

mojo/public/platform/native/mgl_signal_sync_point_thunks.c

+// 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.
+
+#include "mojo/public/platform/native/mgl_signal_sync_point_thunks.h"
+
+#include <assert.h>
+
+#include "mojo/public/platform/native/thunk_export.h"
+
+static struct MGLSignalSyncPointThunks g_signal_sync_point_thunks = {0};
+
+void MGLSignalSyncPoint(uint32_t sync_point,
+                        MGLSignalSyncPointCallback callback,
+                        void* closure) {
+  assert(g_signal_sync_point_thunks.MGLSignalSyncPoint);
+  g_signal_sync_point_thunks.MGLSignalSyncPoint(sync_point, callback, closure);
+}
+
+THUNK_EXPORT size_t MojoSetMGLSignalSyncPointThunks(
+    const struct MGLSignalSyncPointThunks* mgl_signal_sync_point_thunks) {
+  if (mgl_signal_sync_point_thunks->size >= sizeof(g_signal_sync_point_thunks))
+    g_signal_sync_point_thunks = *mgl_signal_sync_point_thunks;
+  return sizeof(g_signal_sync_point_thunks);
+}