Display histograms from subprocesses in chrome://histograms

base/metrics/sparse_histogram.cc

Index: base/metrics/sparse_histogram.cc
diff --git a/base/metrics/sparse_histogram.cc b/base/metrics/sparse_histogram.cc
index e2bf4bfed18a949d8a95177287f9c432541c731a..f9529f7431c926233cb23abbe20922de4cf990d4 100644
--- a/base/metrics/sparse_histogram.cc
+++ b/base/metrics/sparse_histogram.cc
@@ -146,14 +146,16 @@ bool SparseHistogram::AddSamplesFromPickle(PickleIterator* iter) {
   return samples_->AddFromPickle(iter);
 }
 
-void SparseHistogram::WriteHTMLGraph(std::string* output) const {
+void SparseHistogram::WriteHTMLGraph(const HistogramSamples* snapshot,
+                                     std::string* output) const {
   output->append("<PRE>");
-  WriteAsciiImpl(true, "<br>", output);
+  WriteAsciiImpl(true, "<br>", snapshot, output);
   output->append("</PRE>");
 }
 
-void SparseHistogram::WriteAscii(std::string* output) const {
-  WriteAsciiImpl(true, "\n", output);
+void SparseHistogram::WriteAscii(const HistogramSamples* snapshot,
+                                 std::string* output) const {
+  WriteAsciiImpl(true, "\n", snapshot, output);
 }
 
 bool SparseHistogram::SerializeInfoImpl(Pickle* pickle) const {
@@ -211,9 +213,15 @@ void SparseHistogram::GetCountAndBucketData(Count* count,
 
 void SparseHistogram::WriteAsciiImpl(bool graph_it,
                                      const std::string& newline,
+                                     const HistogramSamples* snapshot,
                                      std::string* output) const {
-  // Get a local copy of the data so we are consistent.
-  std::unique_ptr<HistogramSamples> snapshot = SnapshotSamples();
+  std::unique_ptr<HistogramSamples> local_snapshot;
+  if (!snapshot) {
+    // Get local (stack) copies of all effectively volatile class data so
+    // that we are consistent across our output activities.
+    local_snapshot = SnapshotSamples();
+    snapshot = local_snapshot.get();
+  }
   Count total_count = snapshot->TotalCount();
   double scaled_total_count = total_count / 100.0;