Add support for persistent sparse histograms.

base/metrics/histogram.cc

Index: base/metrics/histogram.cc
diff --git a/base/metrics/histogram.cc b/base/metrics/histogram.cc
index ea4f8167fa28cc7098ce233cddbd7ed77fd23a77..f89d770b88443e630ffdfd0efa56e90c35f55969 100644
--- a/base/metrics/histogram.cc
+++ b/base/metrics/histogram.cc
@@ -128,9 +128,7 @@ class Histogram::Factory {
 
   // Perform any required datafill on the just-created histogram.  If
   // overridden, be sure to call the "super" version.

[Alexei Svitkine (slow), 2016/03/14 18:48:59]

Nit: Remove second sentence, since super version is a no-op.

[bcwhite, 2016/03/15 00:00:21]

It is, but there is no guarantee it will always remain that way, or be true if
it were to be derived from a derived class.

-  virtual void FillHistogram(HistogramBase* histogram) {
-    histogram->SetFlags(flags_);
-  }
+  virtual void FillHistogram(HistogramBase* histogram) {}
 
   // These values are protected (instead of private) because they need to
   // be accessible to methods of sub-classes in order to avoid passing
@@ -172,7 +170,7 @@ HistogramBase* Histogram::Factory::Build() {
     }
 
     // Try to create the histogram using a "persistent" allocator. As of
-    // 2015-01-14, the availability of such is controlled by a base::Feature
+    // 2016-02-25, the availability of such is controlled by a base::Feature
     // that is off by default. If the allocator doesn't exist or if
     // allocating from it fails, code below will allocate the histogram from
     // the process heap.
@@ -181,7 +179,6 @@ HistogramBase* Histogram::Factory::Build() {
     PersistentMemoryAllocator* allocator =
         GetPersistentHistogramMemoryAllocator();
     if (allocator) {
-      flags_ |= HistogramBase::kIsPersistent;
       tentative_histogram = AllocatePersistentHistogram(
           allocator,
           histogram_type_,
@@ -200,6 +197,7 @@ HistogramBase* Histogram::Factory::Build() {
       DCHECK(!allocator);  // Shouldn't have failed.
       flags_ &= ~HistogramBase::kIsPersistent;
       tentative_histogram = HeapAlloc(registered_ranges);
+      tentative_histogram->SetFlags(flags_);

[Alexei Svitkine (slow), 2016/03/14 18:49:00]

Nit: Put this inside HeapAlloc()

[bcwhite, 2016/03/15 00:00:21]

It would have to be done inside every HeapAlloc and the purpose of those many 
methods is only to allocate and return; no modifications.

[Alexei Svitkine (slow), 2016/03/16 15:25:43]

To me, it seem a small implementation detail that flags_ is set via a separate
call vs. passed to the ctor. As such, it would be better to abstract it into
HeapAlloc() - since I see that function as allocating the object with the state
specified in member fields (i.e. flags_ is no different than name_ or maximum_).
So I think it's better to have SetFlags() be in HeapAlloc(). And maybe in a
future CL to event make it a ctor param.

[bcwhite, 2016/03/16 15:42:48]

I was thinking the same: It would be best as a ctor param.  Will do.

     }
 
     FillHistogram(tentative_histogram);