Wires up MouseEvent changed flags on windows.

ui/base/win/events_win.cc

 // Copyright (c) 2012 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 <windowsx.h>
 
 #include "ui/base/events.h"
 
 #include "base/logging.h"
 #include "base/time.h"
@@ skipping 93 matching lines @@
     flags |= (win_flags & MK_SHIFT) ? EF_SHIFT_DOWN : 0;
     flags |= (win_flags & MK_CONTROL) ? EF_CONTROL_DOWN : 0;
   }
 
   return flags;
 }
 
 // Returns a mask corresponding to the set of pressed mouse buttons.
 // This includes the button of the given message, even if it is being released.
 int MouseStateFlagsFromNative(const base::NativeEvent& native_event) {
-  // TODO(msw): ORing the pressed/released button into the flags is _wrong_.
-  // It makes it impossible to tell which button was modified when multiple
-  // buttons are/were held down. Instead, we need to track the modified button
-  // independently and audit event consumers to do the right thing.
   int win_flags = GetNativeMouseKey(native_event);
 
   // Client mouse messages provide key states in their WPARAMs.
   if (IsClientMouseEvent(native_event))
     win_flags |= GET_KEYSTATE_WPARAM(native_event.wParam);
 
   int flags = 0;
   flags |= (win_flags & MK_LBUTTON) ? EF_LEFT_MOUSE_BUTTON : 0;
   flags |= (win_flags & MK_MBUTTON) ? EF_MIDDLE_MOUSE_BUTTON : 0;
   flags |= (win_flags & MK_RBUTTON) ? EF_RIGHT_MOUSE_BUTTON : 0;
@@ skipping 91 matching lines @@
   return KeyboardCodeForWindowsKeyCode(native_event.wParam);
 }
 
 bool IsMouseEvent(const base::NativeEvent& native_event) {
   return IsClientMouseEvent(native_event) ||
          IsNonClientMouseEvent(native_event);
 }
 
 int GetChangedMouseButtonFlagsFromNative(
     const base::NativeEvent& native_event) {
-  // TODO(sky): implement me.
+  switch (GetNativeMouseKey(native_event)) {
+    case MK_LBUTTON:
+      return EF_LEFT_MOUSE_BUTTON;
+    case MK_MBUTTON:
+      return EF_MIDDLE_MOUSE_BUTTON;
+    case MK_RBUTTON:
+      return EF_RIGHT_MOUSE_BUTTON;
+    // TODO: add support for MK_XBUTTON1.
+    default:
+      break;
+  }
   return 0;
 }
 
 int GetMouseWheelOffset(const base::NativeEvent& native_event) {
   DCHECK(native_event.message == WM_MOUSEWHEEL);
   return GET_WHEEL_DELTA_WPARAM(native_event.wParam);
 }
 
 int GetTouchId(const base::NativeEvent& xev) {
   NOTIMPLEMENTED();
@@ skipping 95 matching lines @@
 // Windows emulates mouse messages for touch events.
 bool IsMouseEventFromTouch(UINT message) {
   return (message == WM_MOUSEMOVE ||
       message == WM_LBUTTONDOWN || message == WM_LBUTTONUP ||
       message == WM_RBUTTONDOWN || message == WM_RBUTTONUP) &&
       (GetMessageExtraInfo() & MOUSEEVENTF_FROMTOUCH) ==
       MOUSEEVENTF_FROMTOUCH;
 }
 
 }  // namespace ui