X11: Better timestamp handling for _NET_ACTIVE_WINDOW

ui/events/platform/x11/x11_event_source.h

 // 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.
 
 #ifndef UI_EVENTS_PLATFORM_X11_X11_EVENT_SOURCE_H_
 #define UI_EVENTS_PLATFORM_X11_X11_EVENT_SOURCE_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 44 matching lines @@
   // For those that know X11, this is really a wrapper around XWindowEvent
   // which still makes sure the preempted event is dispatched instead of
   // dropped on the floor. This method exists because mapping a window is
   // asynchronous (and we receive an XEvent when mapped), while there are also
   // functions which require a mapped window.
   void BlockUntilWindowMapped(XID window);
 
   XDisplay* display() { return display_; }
   Time last_seen_server_time() const { return last_seen_server_time_; }
 
+  // Explicitly asks the X11 server for the current timestamp, and updates
+  // last_seen_server_time with this value.

[sky, 2016/05/12 15:01:16]

nit: generally when referring to fields and parameters wrap it in |s, eg
|last_seen_server_time_|

+  Time UpdateLastSeenServerTime();
+
   void StopCurrentEventStream();
   void OnDispatcherListChanged();
 
  protected:
   // Extracts cookie data from |xevent| if it's of GenericType, and dispatches
   // the event. This function also frees up the cookie data after dispatch is
   // complete.
   void ExtractCookieDataDispatchEvent(XEvent* xevent);
 
   // Handles updates after event has been dispatched.
@@ skipping 15 matching lines @@
   bool continue_stream_ = true;
 
   std::unique_ptr<X11HotplugEventHandler> hotplug_event_handler_;
 
   DISALLOW_COPY_AND_ASSIGN(X11EventSource);
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_PLATFORM_X11_X11_EVENT_SOURCE_H_