Add thunks for wait set operations, together with public tests.

mojo/public/c/system/tests/time_unittest.cc

 // 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.
 
 // This file tests the C time API (the functions declared in
 // mojo/public/c/system/time.h).
 
 #include "mojo/public/c/system/time.h"
+
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace {
 
 TEST(TimeTest, GetTimeTicksNow) {
   const MojoTimeTicks start = MojoGetTimeTicksNow();
   EXPECT_NE(static_cast<MojoTimeTicks>(0), start)
       << "MojoGetTimeTicksNow should return nonzero value";
 }
 
 }  // namespace