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>
 
 #include "base/macros.h"
 #include "ui/events/events_export.h"
 #include "ui/gfx/x/x11_types.h"
 
 using Time = unsigned long;
 using XEvent = union _XEvent;
 using XID = unsigned long;
+using XWindow = unsigned long;
 
 namespace ui {
 
 class X11HotplugEventHandler;
 
 // Responsible for notifying X11EventSource when new XEvents are available and
 // processing/dispatching XEvents. Implementations will likely be a
 // PlatformEventSource.
 class X11EventSourceDelegate {
  public:
@@ skipping 26 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.
+  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.
   void PostDispatchEvent(XEvent* xevent);
 
  private:
   static X11EventSource* instance_;
 
   X11EventSourceDelegate* delegate_;
 
   // The connection to the X11 server used to receive the events.
   XDisplay* display_;
 
   // The last timestamp seen in an XEvent.
   Time last_seen_server_time_;
 
+  // State necessary for UpdateLastSeenServerTime
+  bool dummy_initialized_;
+  XWindow dummy_window_;
+  XAtom dummy_atom_;
+
   // Keeps track of whether this source should continue to dispatch all the
   // available events.
   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_