Add histograms to track which histograms are going negative and why.

base/metrics/persistent_sample_map.cc

Index: base/metrics/persistent_sample_map.cc
diff --git a/base/metrics/persistent_sample_map.cc b/base/metrics/persistent_sample_map.cc
index af09f97702ea86f7371deb7b8a18e47fbeaca471..cbb7974c5750e1c82d08cf0d80a37a29680fff82 100644
--- a/base/metrics/persistent_sample_map.cc
+++ b/base/metrics/persistent_sample_map.cc
@@ -139,6 +139,10 @@ void PersistentSampleMap::Accumulate(Sample value, Count count) {
   if (reason != MAX_NEGATIVE_SAMPLE_REASONS) {
     UMA_HISTOGRAM_ENUMERATION("UMA.NegativeSamples.Reason", reason,
                               MAX_NEGATIVE_SAMPLE_REASONS);
+    UMA_HISTOGRAM_CUSTOM_COUNTS("UMA.NegativeSamples.Increment", count, 1,
+                                1 << 30, 100);
+    UMA_HISTOGRAM_SPARSE_SLOWLY("UMA.NegativeSamples.Histogram",
+                                static_cast<int32_t>(id()));
   }
 #endif
   IncreaseSumAndCount(strict_cast<int64_t>(count) * value, count);