Fix and reenable flaky PreviewsUKMObserverTest

This test fails and sometimes flakes because the statistics recorder is
interfering with the HistogramTester's init, causing no new entries to be
populated during the test.

Bug: 1002223
Change-Id: Ia7cfe227a5be11c2f8e7826d3377fb6b79952cd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1946843Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721136}

chrome/browser/page_load_metrics/observers/previews_ukm_observer_unittest.cc

@@ -10,7 +10,6 @@
 #include "base/base64.h"
 #include "base/macros.h"
 #include "base/metrics/metrics_hashes.h"
-#include "base/metrics/statistics_recorder.h"
 #include "base/optional.h"
 #include "base/test/scoped_feature_list.h"
 #include "build/build_config.h"
@@ -950,15 +949,7 @@ TEST_F(PreviewsUKMObserverTest, CheckReportingForFlushMetrics) {
               base::nullopt /* hint_source */);
 }
 
-#if defined(OS_ANDROID) || defined(OS_LINUX)
-// Flaky. https://crbug.com/1002223
-#define MAYBE_TestPageEndReasonUMA DISABLED_TestPageEndReasonUMA
-#else
-#define MAYBE_TestPageEndReasonUMA TestPageEndReasonUMA
-#endif
-TEST_F(PreviewsUKMObserverTest, MAYBE_TestPageEndReasonUMA) {
-  std::unique_ptr<base::StatisticsRecorder> recorder(
-      base::StatisticsRecorder::CreateTemporaryForTesting());
+TEST_F(PreviewsUKMObserverTest, TestPageEndReasonUMA) {
   base::HistogramTester histogram_tester;
 
   // No preview: