BackForwardCacheMetricsTest: Disable flaky test

TimeRecordedWhenRendererIsKilled is flaky under TSan:
https://ci.chromium.org/ui/p/chromium/builders/ci/Linux%20TSan%20Tests/69878/overview

Bug: 1255492
Change-Id: I346034978badf0ee77ad51c9a7dc3d1e0a361521
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3201100
Owners-Override: Johann Koenig <johannkoenig@google.com>
Reviewed-by: Yuki Yamada <yukiy@chromium.org>
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/main@{#927583}

content/browser/renderer_host/back_forward_cache_metrics_unittest.cc

@@ -178,7 +178,8 @@ TEST_F(BackForwardCacheMetricsTest, TimeRecordedAtStart) {
               testing::ElementsAre(UkmEntry{id3, {{time_away, 0b1000}}}));
 }
 
-TEST_F(BackForwardCacheMetricsTest, TimeRecordedWhenRendererIsKilled) {
+// TODO(crbug.com/1255492): Flaky under TSan.
+TEST_F(BackForwardCacheMetricsTest, DISABLED_TimeRecordedWhenRendererIsKilled) {
   // Need to enable back-forward cache to make sure a page is put into the
   // cache.
   base::test::ScopedFeatureList scoped_feature_list;