Merge multiple histogram snapshots into single one for reporting.

base/metrics/sample_map.cc

Index: base/metrics/sample_map.cc
diff --git a/base/metrics/sample_map.cc b/base/metrics/sample_map.cc
index 437439e45384279803fb0b4af92229971ab664aa..5798dbc7eb26a6d5e206525e8054a576e8eb68c6 100644
--- a/base/metrics/sample_map.cc
+++ b/base/metrics/sample_map.cc
@@ -38,6 +38,11 @@ Count SampleMap::TotalCount() const {
   return count;
 }
 
+void SampleMap::Negate() {
+  for (auto& entry : sample_counts_)

[Alexei Svitkine (slow), 2015/12/04 18:30:11]

Nit: const auto&

[bcwhite, 2015/12/08 17:32:18]

Can't be "const" because it's being changed.

+    entry.second = -entry.second;
+}
+
 scoped_ptr<SampleCountIterator> SampleMap::Iterator() const {
   return scoped_ptr<SampleCountIterator>(new SampleMapIterator(sample_counts_));
 }