Support saving browser metrics to disk and reading them during next run.

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 42dbac225d830838db4f998020265ec8fe03e380..83bd5feaa85ca9a0c51414ca0ed3f6f4bb5ba969 100644
--- a/base/metrics/histogram_snapshot_manager.h
+++ b/base/metrics/histogram_snapshot_manager.h
@@ -40,18 +40,28 @@ class BASE_EXPORT HistogramSnapshotManager {
   // |Histogram::kNoFlags|. Though any "forward" iterator will work, the
   // histograms over which it iterates *must* remain valid until this method
   // returns; the iterator cannot deallocate histograms once it iterates past
-  // them.
+  // them and FinishDeltas() has been called after.  StartDeltas() must be
+  // called before.
   template <class ForwardHistogramIterator>
-  void PrepareDeltas(ForwardHistogramIterator begin,
-                     ForwardHistogramIterator end,
-                     HistogramBase::Flags flags_to_set,
-                     HistogramBase::Flags required_flags) {
-    StartDeltas();
+  void PrepareDeltasWithoutStartFinish(ForwardHistogramIterator begin,
+                                       ForwardHistogramIterator end,
+                                       HistogramBase::Flags flags_to_set,
+                                       HistogramBase::Flags required_flags) {
     for (ForwardHistogramIterator it = begin; it != end; ++it) {
       (*it)->SetFlags(flags_to_set);
       if (((*it)->flags() & required_flags) == required_flags)
         PrepareDelta(*it);
     }
+  }
+
+  // As above but also calls StartDeltas() and FinishDeltas().
+  template <class ForwardHistogramIterator>
+  void PrepareDeltas(ForwardHistogramIterator begin,
+                     ForwardHistogramIterator end,
+                     HistogramBase::Flags flags_to_set,
+                     HistogramBase::Flags required_flags) {
+    StartDeltas();
+    PrepareDeltasWithoutStartFinish(begin, end, flags_to_set, required_flags);
     FinishDeltas();
   }
 
@@ -62,6 +72,9 @@ class BASE_EXPORT HistogramSnapshotManager {
   // until FinishDeltas() completes. PrepareAbsolute() works the same
   // but assumes there were no previous logged values and no future deltas
   // will be created (and thus can work on read-only histograms).
+  // PrepareFinalDelta() works like PrepareDelta() except that it does
+  // not update the previous logged values and can thus be used with
+  // read-only files.
   // Use Prepare*TakingOwnership() if it is desireable to have this class
   // automatically delete the histogram once it is "finished".
   void StartDeltas();
@@ -70,6 +83,8 @@ class BASE_EXPORT HistogramSnapshotManager {
   void PrepareAbsolute(const HistogramBase* histogram);
   void PrepareAbsoluteTakingOwnership(
       std::unique_ptr<const HistogramBase> histogram);
+  void PrepareFinalDeltaTakingOwnership(
+      std::unique_ptr<const HistogramBase> histogram);
   void FinishDeltas();
 
  private: