MacViews: Implement InputMethodMac::CancelComposition.

ui/base/ime/input_method_mac.mm

 // 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/input_method_mac.h"
 
+#import <Cocoa/Cocoa.h>
+
 namespace ui {
 
 InputMethodMac::InputMethodMac(internal::InputMethodDelegate* delegate) {
   SetDelegate(delegate);
 }
 
 InputMethodMac::~InputMethodMac() {
 }
 
 bool InputMethodMac::OnUntranslatedIMEMessage(const base::NativeEvent& event,
                                               NativeEventResult* result) {
   return false;
 }
 
 void InputMethodMac::DispatchKeyEvent(ui::KeyEvent* event) {
   // This is used on Mac only to dispatch events post-IME.
   ignore_result(DispatchKeyEventPostIME(event));
 }
 
 void InputMethodMac::OnCaretBoundsChanged(const TextInputClient* client) {
 }
 
 void InputMethodMac::CancelComposition(const TextInputClient* client) {
+  if (!IsTextInputClientFocused(client))
+    return;
+
+  [[NSTextInputContext currentInputContext] discardMarkedText];
 }
 
 bool InputMethodMac::IsCandidatePopupOpen() const {
   // There seems to be no way to tell if a candidate window is open.
   return false;
 }
 
 }  // namespace ui