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

components/metrics/file_metrics_provider.cc

Index: components/metrics/file_metrics_provider.cc
diff --git a/components/metrics/file_metrics_provider.cc b/components/metrics/file_metrics_provider.cc
index 36a60df799006f9684477f5f63ce7be256ed5948..65284556c7dbbdb3147823e83134d238179209b3 100644
--- a/components/metrics/file_metrics_provider.cc
+++ b/components/metrics/file_metrics_provider.cc
@@ -185,10 +185,11 @@ void FileMetricsProvider::RecordHistogramSnapshotsFromFile(
     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.

[Ilya Sherman, 2016/04/20 17:10:33]

To clarify: What happens to the actual backing file?  Are the file contents
themselves discarded after reporting is complete?  If not, what mechanism is in
place to ensure that we don't doubly upload data that was persisted to a file?

[bcwhite, 2016/04/22 15:18:05]

The backing file is atomically overwritten (using the ImportantFileWriter) upon
each exit.

The FileMetricsProvider has logic that prevents multiple reads of the same
content by persistently tracking the last-read time and comparing it against the
current last-modified time of the file.

+    snapshot_manager->PrepareDeltaTakingOwnership(std::move(histogram));
     ++histogram_count;
   }