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

base/metrics/persistent_histogram_allocator.h

Index: base/metrics/persistent_histogram_allocator.h
diff --git a/base/metrics/persistent_histogram_allocator.h b/base/metrics/persistent_histogram_allocator.h
index b2038c4314c704f572771648b76410fdcffd03b0..f5a4b48f51ca3ec7aff28767eb13b4cda875913c 100644
--- a/base/metrics/persistent_histogram_allocator.h
+++ b/base/metrics/persistent_histogram_allocator.h
@@ -241,6 +241,16 @@ class BASE_EXPORT GlobalHistogramAllocator
   // in other ways.
   static std::unique_ptr<GlobalHistogramAllocator> ReleaseForTesting();
 
+  // Stores a pathname to which the contents of this allocator should be saved
+  // in order to persist the data for a later use.
+  void SetPersistentLocation(FilePath location);

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

nit: Are FilePaths usually passed by copy or by const ref?

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

I've seen it both ways.  If I make this a ref then I have to do a copy during
the implementation whereas it is currently std::move'd.

I think this is more efficient because FilePaths are often created on-the-fly
and so the compiler can just move the result of that creation into the
parameter, perhaps without ever needing to create the actual object in memory.

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

There was recently a thread on chromium-dev about a similar topic, but for
strings [1].  The conclusion there was to prefer passing by const-ref.  I'd
think the same would apply to FilePath params.  Does that make sense?

[1]
https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/ZoAfW7GXUVI

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

Interesting.  Better to pass-by-ref for maintainability reasons.  Done.

+
+  // Writes the internal data to a previously set location. This is generally
+  // called when a process is exiting from a section of code that may not know
+  // the filesystem. The data is written in an atomic manner. The return value
+  // indicates success.
+  bool WritePersistentLocation();

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

nit: Perhaps "WriteTo" rather than just "Write"?

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

Done.

+
  private:
   friend class StatisticsRecorder;
 
@@ -259,6 +269,9 @@ class BASE_EXPORT GlobalHistogramAllocator
   // iterator to continue the work.
   Iterator import_iterator_;
 
+  // The location to which the data should be persisted.
+  FilePath persistent_location_;

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

nit: Can this be const?

[bcwhite, 2016/04/19 16:33:37]

Const values have to be set in the constructor.  This isn't because that
information isn't known until the higher layers get started.

+
   DISALLOW_COPY_AND_ASSIGN(GlobalHistogramAllocator);
 };