Use correct timestamp for generated mouse events on Mac.

ui/base/cocoa/tool_tip_base_view.mm

 // Copyright 2015 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.
 
 #import "ui/base/cocoa/tool_tip_base_view.h"
 
 #include "base/logging.h"
 
 // Below is the nasty tooltip stuff -- copied from WebKit's WebHTMLView.mm
 // with minor modifications for code style and commenting.
@@ skipping 113 matching lines @@
       continue;
     DCHECK(tag == kTrackingRectTag);
     trackingRectOwner_ = nil;
   }
 }
 
 // Sends a fake NSMouseExited event to the view for its current tracking rect.
 - (void)_sendToolTipMouseExited {
   // Nothing matters except window, trackingNumber, and userData.
   int windowNumber = [[self window] windowNumber];
+  NSTimeInterval eventTime = [[NSApp currentEvent] timestamp];
   NSEvent* fakeEvent = [NSEvent enterExitEventWithType:NSMouseExited
                                               location:NSZeroPoint
                                          modifierFlags:0
-                                             timestamp:0
+                                             timestamp:eventTime
                                           windowNumber:windowNumber
                                                context:NULL
                                            eventNumber:0
                                         trackingNumber:kTrackingRectTag
                                               userData:trackingRectUserData_];
   [trackingRectOwner_ mouseExited:fakeEvent];
 }
 
 // Sends a fake NSMouseEntered event to the view for its current tracking rect.
 - (void)_sendToolTipMouseEntered {
   // Nothing matters except window, trackingNumber, and userData.
   int windowNumber = [[self window] windowNumber];
+  NSTimeInterval eventTime = [[NSApp currentEvent] timestamp];
   NSEvent* fakeEvent = [NSEvent enterExitEventWithType:NSMouseEntered
                                               location:NSZeroPoint
                                          modifierFlags:0
-                                             timestamp:0
+                                             timestamp:eventTime
                                           windowNumber:windowNumber
                                                context:NULL
                                            eventNumber:0
                                         trackingNumber:kTrackingRectTag
                                               userData:trackingRectUserData_];
   [trackingRectOwner_ mouseEntered:fakeEvent];
 }
 
 // Sets the view's current tooltip, to be displayed at the current mouse
 // location. (This does not make the tooltip appear -- as usual, it only
@@ skipping 25 matching lines @@
 
 // NSView calls this to get the text when displaying the tooltip.
 - (NSString *)view:(NSView *)view
   stringForToolTip:(NSToolTipTag)tag
              point:(NSPoint)point
           userData:(void *)data {
   return [[toolTip_ copy] autorelease];
 }
 
 @end