Replace DCHECK() upon unexpected x11 event time with a workaround

ui/events/x/events_x_utils.cc

Index: ui/events/x/events_x_utils.cc
diff --git a/ui/events/x/events_x_utils.cc b/ui/events/x/events_x_utils.cc
index aab272373068313cf5ae95cd5fbb27b34fa12287..a0abd325046414396427db72b8a84b65bbf5153e 100644
--- a/ui/events/x/events_x_utils.cc
+++ b/ui/events/x/events_x_utils.cc
@@ -306,11 +306,13 @@ bool GetGestureTimes(const XEvent& xev, double* start_time, double* end_time) {
   return true;
 }
 
+namespace {
 int64_t g_last_seen_timestamp_ms = 0;
-// accumulated rollover time.
 int64_t g_rollover_ms = 0;
+bool g_bogus_x11_timestamps = false;
 base::LazyInstance<std::unique_ptr<base::TickClock>>::Leaky g_tick_clock =
     LAZY_INSTANCE_INITIALIZER;
+}  // namespace
 
 // Takes Xlib Time and returns a time delta that is immune to timer rollover.
 // This function is not thread safe as we do not use a lock.
@@ -320,6 +322,11 @@ base::TimeDelta TimeDeltaFromXEventTime(Time timestamp) {
   if (!timestamp)
     return base::TimeDelta();
 
+  if (g_bogus_x11_timestamps) {
+    return base::TimeDelta::FromMilliseconds(
+        (base::TimeTicks::Now() - base::TimeTicks()).InMilliseconds());

[majidvp, 2016/05/17 13:27:53]

Please use ui::EventTimeForNow() instead.

I don't think we should limit these timestamps to milliseconds resolution if we
are using TimeTicks::Now().

[caseq, 2016/05/20 23:41:33]

Done.

+  }
+
   // If this is the first event that we get, assume the time stamp roll-over
   // might have happened before the process was started.
   // Register a rollover if the distance between last timestamp and current one
@@ -343,11 +350,13 @@ base::TimeDelta TimeDeltaFromXEventTime(Time timestamp) {
 
   g_rollover_ms = now_ms & ~static_cast<int64_t>(UINT32_MAX);
   uint32_t delta = static_cast<uint32_t>(now_ms - timestamp);
-  // If using a mock clock, all bets are off -- in some tests, actual X11 events
-  // come through with real timestamps.
-  DCHECK(delta < 60 * 1000 || g_tick_clock.Get() != nullptr)
-      << "Unexpected X11 event time, now: " << now_ticks
-      << " event at: " << timestamp;
+  if (delta > 60 * 1000 && !g_tick_clock.Get()) {
+    // x11 timestamps don't seem to be using the same time base as TimeTicks,
+    // so ignore them altogether and always use current time instead.
+    delta = 0;
+    g_bogus_x11_timestamps = true;
+    LOG(WARNING) << "Unexpected x11 timestamps, will use browser time instead.";

[majidvp, 2016/05/17 13:27:53]

I wonder if it makes sense to also collect UMA metrics for this.

I suspect this should not happen very often. But if this turns out to be a large
percentage of cases then it can impact the way we interpret our event latency
telemetry metrics on Linux.

+tdresser@ in case he has opinion one way or another.

[caseq, 2016/05/20 23:41:33]

Done.

+  }
   return base::TimeDelta::FromMilliseconds(now_ms - delta);
 }
 
@@ -813,6 +822,7 @@ void ResetTimestampRolloverCountersForTesting(
     std::unique_ptr<base::TickClock> tick_clock) {
   g_last_seen_timestamp_ms = 0;
   g_rollover_ms = 0;
+  g_bogus_x11_timestamps = false;
   g_tick_clock.Get() = std::move(tick_clock);
 }