Add histogram for native allocations

Shows up in traces.txt, sample output:
Histogram of native allocation 0:4315,131072:33,1179648:3 bucket size 131072
Histogram of native free 0:995,131072:19,1179648:2 bucket size 131072

Bug: 28680116

Change-Id: I584eb85709d3ae28f5142a01af11075a2a6436d7
diff --git a/runtime/base/histogram.h b/runtime/base/histogram.h
index bcb7b3b..0e3bc8e 100644
--- a/runtime/base/histogram.h
+++ b/runtime/base/histogram.h
@@ -85,6 +85,10 @@
     return max_value_added_;
   }
 
+  Value BucketWidth() const {
+    return bucket_width_;
+  }
+
   const std::string& Name() const {
     return name_;
   }