Fix UB in events_unittests

Use ui::test::ScopedEventTestTickClock instead of
SetEventTickClockForTesting.

EventsXTest did not call SetEventTickClockForTesting(nullptr) and,
eventually, GestureProviderAuraTest suffered from that.

It's better to use RAII idiom in this case because RAII makes impossible
to forget to call SetEventTickClockForTesting(nullptr).

Change-Id: I80b079ef37a77618d76f14912f7d6c40d39ab66b
Reviewed-on: https://chromium-review.googlesource.com/1030108
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569607}

ui/events/x/events_x_unittest.cc

@@ -21,6 +21,7 @@
 #include "ui/events/event_utils.h"
 #include "ui/events/test/events_test_utils.h"
 #include "ui/events/test/events_test_utils_x11.h"
+#include "ui/events/test/scoped_event_test_tick_clock.h"
 #include "ui/events/x/events_x_utils.h"
 #include "ui/gfx/geometry/point.h"
 #include "ui/gfx/x/x11.h"
@@ -554,9 +555,8 @@ TEST_F(EventsXTest, TimestampRolloverAndAdjustWhenDecreasing) {
   XEvent event;
   InitButtonEvent(&event, true, gfx::Point(5, 10), 1, 0);
 
-  base::SimpleTestTickClock clock;
+  test::ScopedEventTestTickClock clock;
   clock.SetNowTicks(TimeTicksFromMillis(0x100000001));
-  SetEventTickClockForTesting(&clock);
   ResetTimestampRolloverCountersForTesting();
 
   event.xbutton.time = 0xFFFFFFFF;
@@ -574,9 +574,8 @@ TEST_F(EventsXTest, NoTimestampRolloverWhenMonotonicIncreasing) {
   XEvent event;
   InitButtonEvent(&event, true, gfx::Point(5, 10), 1, 0);
 
-  base::SimpleTestTickClock clock;
+  test::ScopedEventTestTickClock clock;
   clock.SetNowTicks(TimeTicksFromMillis(10));
-  SetEventTickClockForTesting(&clock);
   ResetTimestampRolloverCountersForTesting();
 
   event.xbutton.time = 6;