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

components/metrics/file_metrics_provider.cc

 // Copyright 2016 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 "components/metrics/file_metrics_provider.h"
 
 #include "base/command_line.h"
 #include "base/files/file.h"
 #include "base/files/file_util.h"
 #include "base/files/memory_mapped_file.h"
@@ skipping 167 matching lines @@
     FileInfo* file) {
   DCHECK(thread_checker_.CalledOnValidThread());
   base::PersistentHistogramAllocator::Iterator histogram_iter(
       file->allocator.get());
 
   int histogram_count = 0;
   while (true) {
     scoped_ptr<base::HistogramBase> histogram = histogram_iter.GetNext();
     if (!histogram)
       break;
-    if (file->type == FILE_HISTOGRAMS_ATOMIC)
-      snapshot_manager->PrepareAbsoluteTakingOwnership(std::move(histogram));
-    else
-      snapshot_manager->PrepareDeltaTakingOwnership(std::move(histogram));
+    // The "delta" is calculated, which updates the "logged" counts, even
+    // for "atomic" files that may be read-only. This is possible because
+    // the file contents are copied into read/write memory which is then
+    // discarded after the reporting is complete.
+    snapshot_manager->PrepareDeltaTakingOwnership(std::move(histogram));
     ++histogram_count;
   }
 
   DVLOG(1) << "Reported " << histogram_count << " histograms from "
            << file->path.value();
 }
 
 void FileMetricsProvider::CreateAllocatorForFile(FileInfo* file) {
   DCHECK(!file->allocator);
 
@@ skipping 91 matching lines @@
 
     // Dump all histograms contained within the file to the snapshot-manager.
     RecordHistogramSnapshotsFromFile(snapshot_manager, file.get());
 
     // Update the last-seen time so it isn't read again unless it changes.
     RecordFileAsSeen(file.get());
   }
 }
 
 }  // namespace metrics