Report histogram creation results.

base/metrics/histogram_persistence.cc

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/metrics/histogram_persistence.h"
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram.h"
@@ skipping 169 matching lines @@
     result_histogram->Add(result);
 }
 
 void SetPersistentHistogramMemoryAllocator(
     PersistentMemoryAllocator* allocator) {
   // Releasing or changing an allocator is extremely dangerous because it
   // likely has histograms stored within it. If the backing memory is also
   // also released, future accesses to those histograms will seg-fault.
   CHECK(!g_allocator);
   g_allocator = allocator;
+
+  size_t existing = StatisticsRecorder::NumberOfHistograms();
+  if (existing) {

[Alexei Svitkine (slow), 2016/03/01 16:41:34]

Nit: != 0 for checking numbers.

[bcwhite, 2016/03/02 19:14:19]

Done.

+    DLOG(WARNING) << existing
+                  << " histograms were created before persistence was enabled.";
+  }
 }
 
 PersistentMemoryAllocator* GetPersistentHistogramMemoryAllocator() {
   return g_allocator;
 }
 
 PersistentMemoryAllocator*
 ReleasePersistentHistogramMemoryAllocatorForTesting() {
   PersistentMemoryAllocator* allocator = g_allocator;
   if (!allocator)
@@ skipping 307 matching lines @@
     while (true) {
       HistogramBase* histogram = GetNextPersistentHistogram(g_allocator, &iter);
       if (!histogram)
         break;
       StatisticsRecorder::RegisterOrDeleteDuplicate(histogram);
     }
   }
 }
 
 }  // namespace base