Revert "Disable flaky MetricsWebContentsObserverTest.TimingOrderError"

This reverts commit 255e53df.

Reason for revert: Test is not the root cause of this failure. See https://bugs.chromium.org/p/chromium/issues/detail?id=1011275

Original change's description:
> Disable flaky MetricsWebContentsObserverTest.TimingOrderError
> 
> TBR=csharrison@chromium.org
> 
> Bug: 1011203
> Change-Id: Ie9807b09fb16726f117c02507a3a200d22acfe0f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1839311
> Reviewed-by: Yoichi Osato <yoichio@chromium.org>
> Commit-Queue: Yoichi Osato <yoichio@chromium.org>
> Auto-Submit: Yoichi Osato <yoichio@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#702698}

TBR=yoichio@chromium.org,csharrison@chromium.org

Change-Id: Ie3c883479388584376bd84816ae7e379f4e09ea6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1011203
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841111Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702841}

components/page_load_metrics/browser/metrics_web_contents_observer_unittest.cc

@@ -487,7 +487,7 @@ TEST_F(MetricsWebContentsObserverTest, NullNavigationStartError) {
       page_load_metrics::internal::INVALID_NULL_NAVIGATION_START, 1);
 }
 
-TEST_F(MetricsWebContentsObserverTest, DISABLED_TimingOrderError) {
+TEST_F(MetricsWebContentsObserverTest, TimingOrderError) {
   mojom::PageLoadTiming timing;
   page_load_metrics::InitPageLoadTimingForTest(&timing);
   timing.navigation_start = base::Time::FromDoubleT(1);