Support negative sample values in PersistentHistogramMap.

base/metrics/persistent_sample_map.h

Index: base/metrics/persistent_sample_map.h
diff --git a/base/metrics/persistent_sample_map.h b/base/metrics/persistent_sample_map.h
index 6cde349390db6acfd7cdf5771b847a505e3253af..10be75f22cc3b7ca2261e6d70cd8cb06b4614639 100644
--- a/base/metrics/persistent_sample_map.h
+++ b/base/metrics/persistent_sample_map.h
@@ -15,6 +15,7 @@
 #include <memory>
 
 #include "base/compiler_specific.h"
+#include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
@@ -81,7 +82,12 @@ class BASE_EXPORT PersistentSampleMap : public HistogramSamples {
       HistogramBase::Sample value);
 
  private:
-  enum : HistogramBase::Sample { kAllSamples = -1 };
+  FRIEND_TEST_ALL_PREFIXES(PersistentSampleMapTest, PersistenceTest);
+
+  // Since all values are possible in a SparseHistogram, the value of
+  // kAllSamples is chosen to be something unlikely. In the case that this
+  // value is used within the sample map, it is still safe.
+  enum : HistogramBase::Sample { kAllSamples = -10000 };

[Alexei Svitkine (slow), 2016/04/20 21:51:18]

Can we avoid having this constant completely? For example, having a different
function to return all samples?

[Alexei Svitkine (slow), 2016/04/20 21:51:55]

Or a separate boolean parameter?

[bcwhite, 2016/04/21 10:33:15]

Sure, though the two parameters would generally be mutually exclusive.  I
suppose I could pass a pointer to a Sample and allow "nullptr" to mean "all". 
Nah...  Boolean works better.

Done.

Two functions would be pretty much complete duplicates of each other so I don't
think it's a good idea.

 
   // Imports samples from persistent memory by iterating over all sample
   // records found therein, adding them to the sample_counts_ map. If a