Reland rewrite clipboard write IPC handling to be easier to understand.

ui/base/test/test_clipboard_unittest.cc

 // Copyright 2014 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 "ui/base/test/test_clipboard.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace ui {
 
 struct TestClipboardTraits {
-  static Clipboard* Create() {
-    TestClipboard::UseForCurrentThread();
-    return Clipboard::GetForCurrentThread();
-  }
+  static Clipboard* Create() { return TestClipboard::CreateForCurrentThread(); }
 
   static void Destroy(Clipboard* clipboard) {
     ASSERT_EQ(Clipboard::GetForCurrentThread(), clipboard);
     Clipboard::DestroyClipboardForCurrentThread();
   }
 };
 
 typedef TestClipboardTraits TypesToTest;
 
 }  // namespace ui
 
 #include "ui/base/clipboard/clipboard_test_template.h"