Merge multiple histogram snapshots into single one for reporting.

base/metrics/statistics_recorder.cc

Index: base/metrics/statistics_recorder.cc
diff --git a/base/metrics/statistics_recorder.cc b/base/metrics/statistics_recorder.cc
index 12e448941c848093e1c4396735982b964b00205b..7efe16b10c15596f968a4fcced8ad4ae4d7d9ee4 100644
--- a/base/metrics/statistics_recorder.cc
+++ b/base/metrics/statistics_recorder.cc
@@ -392,6 +392,25 @@ StatisticsRecorder::StatisticsRecorder() {
 }
 
 // static
+void StatisticsRecorder::ResetForTesting() {

[Alexei Svitkine (slow), 2016/02/17 16:21:12]

Please order this same as in the .h file - i.e. above the ctor. Also, can you
move DumpHistogramsToVlog() to below the dtor?

[bcwhite, 2016/02/17 17:58:20]

This is being moved into another CL.

[Alexei Svitkine (slow), 2016/02/17 18:15:16]

So should it be deleted from this CL? Or are you planning to land the other one
first and rebase this one on top?

[bcwhite, 2016/02/17 19:39:50]

The latter.  I think the other one is ready to land now.

+  if (histograms_) {
+    STLDeleteValues(histograms_);
+    delete histograms_;
+    histograms_= nullptr;
+  }
+  if (ranges_) {
+    for (auto& iter : *ranges_) {
+      STLDeleteElements(iter.second);
+      delete iter.second;
+    }
+    delete ranges_;
+    ranges_ = nullptr;
+  }
+  delete callbacks_;
+  callbacks_ = nullptr;

[Alexei Svitkine (slow), 2016/02/17 16:21:12]

A lot of this code is similar to what's done in the dtor.

Can you make a private helper function? I know one difference is whether the
container values are deleted - but you can make a boolean param in the helper
for this.

I would also keep the structure of the dtor and have separate if (histograms_)
STLDelete* stmts for when the boolean specify to delete the contents.

(The reason to consolidate them is to make it less likely for someone update one
but forget the other.)

[bcwhite, 2016/02/17 17:58:20]

This is being moved into another CL.  It turns out that trying to clear the
histograms from the map can actually cause problems during testing because of
static pointers to histograms created in other code.

+}
+
+// static
 void StatisticsRecorder::DumpHistogramsToVlog(void* instance) {
   std::string output;
   StatisticsRecorder::WriteGraph(std::string(), &output);