Report histogram creation results.

base/metrics/persistent_histogram_allocator.cc

Index: base/metrics/persistent_histogram_allocator.cc
diff --git a/base/metrics/persistent_histogram_allocator.cc b/base/metrics/persistent_histogram_allocator.cc
index 1929060d8e5421ff2c64fb6aa00dce3e6a5cf43f..2e4029ef6a60e7f2bd68e67c42f1ad861e9851cf 100644
--- a/base/metrics/persistent_histogram_allocator.cc
+++ b/base/metrics/persistent_histogram_allocator.cc
@@ -179,6 +179,11 @@ void PersistentHistogramAllocator::SetGlobalAllocator(
   // also released, future accesses to those histograms will seg-fault.
   CHECK(!g_allocator);
   g_allocator = allocator.release();
+
+  size_t existing = StatisticsRecorder::GetHistogramCount();
+  DLOG_IF(WARNING, existing)
+      << existing
+      << " histograms were created before persistence was enabled.";
 }
 
 // static