X11: Better timestamp handling for _NET_ACTIVE_WINDOW

ui/events/platform/x11/x11_event_source.cc

Index: ui/events/platform/x11/x11_event_source.cc
diff --git a/ui/events/platform/x11/x11_event_source.cc b/ui/events/platform/x11/x11_event_source.cc
index 0319da91b56843494f6c487d3876f316098142fd..73a8e26213121f9b1cba9821a033bf1cc63979ac 100644
--- a/ui/events/platform/x11/x11_event_source.cc
+++ b/ui/events/platform/x11/x11_event_source.cc
@@ -4,10 +4,12 @@
 
 #include "ui/events/platform/x11/x11_event_source.h"
 
+#include <X11/Xatom.h>
 #include <X11/XKBlib.h>
 #include <X11/Xlib.h>
 
 #include "base/logging.h"
+#include "base/metrics/histogram_macros.h"
 #include "ui/events/devices/x11/device_data_manager_x11.h"
 #include "ui/events/event_utils.h"
 #include "ui/events/platform/platform_event_dispatcher.h"
@@ -126,6 +128,43 @@ void X11EventSource::BlockUntilWindowMapped(XID window) {
   } while (event.type != MapNotify);
 }
 
+static Bool is_property_notify_for_timestamp(Display* display,
+                                             XEvent* event,
+                                             XPointer arg) {
+  return event->type == PropertyNotify &&
+      event->xproperty.window == DefaultRootWindow(display) &&
+      event->xproperty.atom == *(Atom*)arg;
+}
+
+Time X11EventSource::UpdateServerTime() {
+  base::TimeTicks start = base::TimeTicks::Now();
+
+  const char kAtomStr[] = __FILE__ " CHROMIUM_DUMMY_ATOM";

[Elliot Glaysher, 2016/05/06 22:21:10]

Are spaces allowed in atoms? Probably want to change that ' ' to a '_'. What
about the '/' in __FILE__?

[Tom (Use chromium acct), 2016/05/06 22:44:59]

The doc says atom names are ISO Latin-1 strings.  Changed it to something
well-defined just in case

+
+  DCHECK(display_);
+  Window window = DefaultRootWindow(display_);
+  const unsigned char dummyData = 0;
+  Atom dummyAtom = XInternAtom(display_, kAtomStr, False);
+
+  XSelectInput(display_, window, PropertyChangeMask);
+
+  XChangeProperty(display_, window, dummyAtom, XA_INTEGER, 8, PropModeReplace,
+                  &dummyData, 1);
+
+  XEvent event;
+  XIfEvent(display_, &event, is_property_notify_for_timestamp,
+           (XPointer)&dummyAtom);
+
+  XDeleteProperty(display_, window, dummyAtom);
+  XFlush(display_);
+
+  last_seen_server_time_ = event.xproperty.time;
+
+  UMA_HISTOGRAM_TIMES("Event.Latency.X11EventSource.UpdateServerTime",
+                      base::TimeTicks::Now() - start);
+  return last_seen_server_time_;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // X11EventSource, protected