Reorganize histograms for persistence.

base/metrics/histogram_snapshot_manager.h

Index: base/metrics/histogram_snapshot_manager.h
diff --git a/base/metrics/histogram_snapshot_manager.h b/base/metrics/histogram_snapshot_manager.h
index 5a5f2e93e5e88b28c0e3898198cf2ae646f636c6..484287c2ab9f35b27ba53475a19e2beda9a39abb 100644
--- a/base/metrics/histogram_snapshot_manager.h
+++ b/base/metrics/histogram_snapshot_manager.h
@@ -48,10 +48,10 @@ class BASE_EXPORT HistogramSnapshotManager {
 
   // For histograms, track what we've already recorded (as a sample for
   // each histogram) so that we can record only the delta with the next log.
-  std::map<std::string, HistogramSamples*> logged_samples_;
+  std::map<uint64_t, HistogramSamples*> logged_samples_;
 
   // List of histograms found to be corrupt, and their problems.
-  std::map<std::string, int> inconsistencies_;
+  std::map<uint64_t, int> inconsistencies_;
 
   // |histogram_flattener_| handles the logistics of recording the histogram
   // deltas.