Redesigns the text input focus handling.

ui/base/ime/text_input_focus_manager.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/ime/text_input_focus_manager.h"
+
+#include "base/memory/singleton.h"
+
+namespace ui {
+
+TextInputFocusManager::~TextInputFocusManager() {}
+
+TextInputFocusManager* TextInputFocusManager::GetInstance() {
+  return Singleton<TextInputFocusManager>::get();
+}
+
+TextInputClient* TextInputFocusManager::GetFocusedTextInputClient() {
+  return focused_text_input_client_;
+}
+
+void TextInputFocusManager::SetFocusedTextInputClient(
+    TextInputClient* text_input_client) {
+  focused_text_input_client_ = text_input_client;
+}
+
+void TextInputFocusManager::UnsetFocusedTextInputClient(

[msw, 2014/03/11 00:58:50]

Is this function even necessary? A call could easily do this:
if (client == manager->GetFocusedTextInputClient())
  manager->SetFocusedTextInputClient(NULL);

[Yuki, 2014/03/11 15:27:37]

What the client would like to say is "forget me.  good bye."

The implementation about how to forget should be hidden in
TextInputFocusManager.

I think, in general, that it's easier to introduce bugs when we allow (or force)
the client code to check the condition and do the things.  I don't like to trust
the client code too much.

Please consider these three methods as a sort of Get, Add and Remove.  Then it
seems natural to have Unset method here, doesn't it?

[msw, 2014/03/11 23:24:37]

I'm on the fence about this, but I'll concede to your points.

+    TextInputClient* text_input_client) {
+  if (focused_text_input_client_ == text_input_client)
+    focused_text_input_client_ = NULL;
+}
+
+TextInputFocusManager::TextInputFocusManager()
+    : focused_text_input_client_(NULL) {}
+
+}  // namespace ui