SelectionOwner: add support for TIMESTAMP target

ui/events/platform/x11/x11_event_source.h

Index: ui/events/platform/x11/x11_event_source.h
diff --git a/ui/events/platform/x11/x11_event_source.h b/ui/events/platform/x11/x11_event_source.h
index afc05882bf2c65859061438957a8af5cdbb46d54..5b74e6357c8ed7ec8f33d634cd6d9604cab4e1c4 100644
--- a/ui/events/platform/x11/x11_event_source.h
+++ b/ui/events/platform/x11/x11_event_source.h
@@ -13,8 +13,9 @@
 #include "ui/events/events_export.h"
 #include "ui/gfx/x/x11_types.h"
 
-typedef union _XEvent XEvent;
-typedef unsigned long XID;
+using Time = unsigned long;
+using XEvent = union _XEvent;
+using XID = unsigned long;
 
 namespace ui {
 
@@ -59,6 +60,7 @@ class EVENTS_EXPORT X11EventSource {
   void BlockUntilWindowMapped(XID window);
 
   XDisplay* display() { return display_; }
+  Time last_seen_server_time() const { return last_seen_server_time_; }
 
   void StopCurrentEventStream();
   void OnDispatcherListChanged();
@@ -80,6 +82,9 @@ class EVENTS_EXPORT X11EventSource {
   // 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_;
+
   // Keeps track of whether this source should continue to dispatch all the
   // available events.
   bool continue_stream_ = true;