Merge multiple histogram snapshots into single one for reporting.

base/metrics/sample_vector.cc

Index: base/metrics/sample_vector.cc
diff --git a/base/metrics/sample_vector.cc b/base/metrics/sample_vector.cc
index a77cd71d422f574992a1731a37446d1d6bef1372..6c833adeb9d58bc18ca6351c84126cf85b8dff54 100644
--- a/base/metrics/sample_vector.cc
+++ b/base/metrics/sample_vector.cc
@@ -66,6 +66,13 @@ Count SampleVector::TotalCount() const {
   return count;
 }
 
+void SampleVector::Negate() {
+  for (size_t i = 0; i < counts_size_; i++) {
+    subtle::NoBarrier_Store(&counts_[i],
+                            -subtle::NoBarrier_Load(&counts_[i]));
+  }
+}
+
 Count SampleVector::GetCountAtIndex(size_t bucket_index) const {
   DCHECK(bucket_index < counts_size_);
   return subtle::NoBarrier_Load(&counts_[bucket_index]);