Should set the modifiers for web mouse event on Mac.

Source/web/WebInputEventFactoryMac.mm

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2006-2009 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 704 matching lines @@
     if ([event modifierFlags] & NSControlKeyMask)
         modifiers |= WebInputEvent::ControlKey;
     if ([event modifierFlags] & NSShiftKeyMask)
         modifiers |= WebInputEvent::ShiftKey;
     if ([event modifierFlags] & NSAlternateKeyMask)
         modifiers |= WebInputEvent::AltKey;
     if ([event modifierFlags] & NSCommandKeyMask)
         modifiers |= WebInputEvent::MetaKey;
     if ([event modifierFlags] & NSAlphaShiftKeyMask)
         modifiers |= WebInputEvent::CapsLockOn;
-    // TODO(port): Set mouse button states
+
+    // The return value of 1 << 0 corresponds to the left mouse button,
+    // 1 << 1 corresponds to the right mouse button,
+    // 1 << n, n >= 2 correspond to other mouse buttons.
+    NSUInteger pressedButtons = [NSEvent pressedMouseButtons];
+
+    if (pressedButtons & (1 << 0))
+        modifiers |= WebInputEvent::LeftButtonDown;
+    if (pressedButtons & (1 << 1))
+        modifiers |= WebInputEvent::RightButtonDown;
+    if (pressedButtons & (1 << 2))
+        modifiers |= WebInputEvent::MiddleButtonDown;
 
     return modifiers;
 }
 
 static inline void setWebEventLocationFromEventInView(WebMouseEvent* result,
                                                       NSEvent* event,
                                                       NSView* view) {
     NSPoint windowLocal = [event locationInWindow];
 
     NSPoint screenLocal = [[view window] convertBaseToScreen:windowLocal];
@@ skipping 404 matching lines @@
         break;
     default:
         ASSERT_NOT_REACHED();
         result.type = WebInputEvent::Undefined;
     }
 
     return result;
 }
 
 } // namespace blink