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 78fb90a1211c5f6b4e69330517e4197aef496c58..ee9ec22c06b7a7879b6fb8fe86674c3c61a74537 100644
--- a/components/metrics/file_metrics_provider.cc
+++ b/components/metrics/file_metrics_provider.cc
@@ -183,7 +183,7 @@ void FileMetricsProvider::RecordHistogramSnapshotsFromFile(
     if (!histogram)
       break;
     if (file->type == FILE_HISTOGRAMS_ATOMIC)
-      snapshot_manager->PrepareAbsoluteTakingOwnership(std::move(histogram));
+      snapshot_manager->PrepareDifferenceTakingOwnership(std::move(histogram));

[Ilya Sherman, 2016/04/19 00:57:46]

So, why is it important here that we take the (idempotent) difference, rather
than a non-idempotent delta?  It seems kind of wrong to me, but maybe there's
something I'm just not thinking of properly.

[bcwhite, 2016/04/19 16:33:38]

Because in some situations, the data may be read-only.  SetupMetrics is not
writable because it's created by an Administrator process.

Though...  The current implementation is to copy the file into RAM anyway so any
written changes would be discarded anyway.  Using PrepareDelta() could cause a
problem if we ever go back to using a memory-mapped file instead of RAM copy,
but I guess I could back-out the PrepareDifference changes.

Thoughts?

[Ilya Sherman, 2016/04/20 07:12:14]

I'd prefer to back-out the PrepareDifference changes for now if they're not
needed.

For a read-only file, I don't understand how PrepareDelta() could possibly be
different than PrepareAbsolute() -- if it's strictly read-only, then there can't
have been any deltas performed on it.  Right?

[bcwhite, 2016/04/20 16:47:45]

Done.
Uploads could have been done on the data _before_ it was written out to disk. 
It's dumped as a single memory block with both "active" and "logged" counts held
side-by-side.

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