getRuntimeStat() support (ART).

Export some runtime stats (currently GC stats) via
VMDebug.getRuntimeStat().

Added several new GC stats such as blocking GC counts and GC count
histograms.

Bug: 19825248
Change-Id: I8ece9ed241dc3982dfd983d7159090ba82940dce
diff --git a/runtime/base/histogram.h b/runtime/base/histogram.h
index 78f6e1c..c312fb2 100644
--- a/runtime/base/histogram.h
+++ b/runtime/base/histogram.h
@@ -61,6 +61,7 @@
   void PrintConfidenceIntervals(std::ostream& os, double interval,
                                 const CumulativeData& data) const;
   void PrintBins(std::ostream& os, const CumulativeData& data) const;
+  void DumpBins(std::ostream& os) const;
   Value GetRange(size_t bucket_idx) const;
   size_t GetBucketCount() const;