Fix StatisticsRecorder to handle re-entry during tests.

base/metrics/statistics_recorder.h

Index: base/metrics/statistics_recorder.h
diff --git a/base/metrics/statistics_recorder.h b/base/metrics/statistics_recorder.h
index 36b2f308af30bf8bc52d23a7ddfd9af69d22ccc7..314600fdb09a7a8531a60355c6047e591ce66304 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -130,10 +130,6 @@ class BASE_EXPORT StatisticsRecorder {
   // histogram. This method is thread safe.
   static OnSampleCallback FindCallback(const std::string& histogram_name);
 
-  // Clears all of the known histograms and resets static variables to a
-  // state that allows a new initialization.
-  static void ResetForTesting();
-
   // Removes a histogram from the internal set of known ones. This can be
   // necessary during testing persistent histograms where the underlying
   // memory is being released.
@@ -166,6 +162,13 @@ class BASE_EXPORT StatisticsRecorder {
   StatisticsRecorder();
   ~StatisticsRecorder();
 
+  // These are copies of everything that existed when the (test) Statistics-
+  // Recorder was created. The global ones have to be moved aside to create a
+  // clean environment.
+  scoped_ptr<HistogramMap> existing_histograms_;
+  scoped_ptr<CallbackMap> existing_callbacks_;
+  scoped_ptr<RangesMap> existing_ranges_;

[Alexei Svitkine (slow), 2016/03/08 20:04:12]

Can this be done in a test-only subclass?

[bcwhite, 2016/03/08 20:23:27]

Potentially.  It'll require turning private members into protected ones.

It's already basically a test-only class except for the one global instance. 
We'll waste 3 pointers worth of memory on that instance.  Still think it's worth
it?

+
   static void Reset();
   static void DumpHistogramsToVlog(void* instance);