MacViews: Implement InputMethodMac::CancelComposition.

ui/base/ime/input_method_mac.mm

Index: ui/base/ime/input_method_mac.mm
diff --git a/ui/base/ime/input_method_mac.mm b/ui/base/ime/input_method_mac.mm
index b03b1620e3f1758361d109a1794036172a841452..e53f06744475a57c05a76c50051f75a527de95a4 100644
--- a/ui/base/ime/input_method_mac.mm
+++ b/ui/base/ime/input_method_mac.mm
@@ -4,6 +4,8 @@
 
 #include "ui/base/ime/input_method_mac.h"
 
+#import <Cocoa/Cocoa.h>
+
 namespace ui {
 
 InputMethodMac::InputMethodMac(internal::InputMethodDelegate* delegate) {
@@ -27,6 +29,10 @@
 }
 
 void InputMethodMac::CancelComposition(const TextInputClient* client) {
+  if (!IsTextInputClientFocused(client))
+    return;
+
+  [[NSTextInputContext currentInputContext] discardMarkedText];
 }
 
 bool InputMethodMac::IsCandidatePopupOpen() const {