Make UMA stats creation available in the Java interface.

Only has counts stats right now but enumeration stats can easily be added in the future if needed.

BUG=webrtc:6313

Review-Url: https://codereview.webrtc.org/2320473002
Cr-Commit-Position: refs/heads/master@{#14146}
diff --git a/webrtc/system_wrappers/source/metrics_default.cc b/webrtc/system_wrappers/source/metrics_default.cc
index 4900454..fd7610b 100644
--- a/webrtc/system_wrappers/source/metrics_default.cc
+++ b/webrtc/system_wrappers/source/metrics_default.cc
@@ -244,6 +244,15 @@
   ptr->Add(sample);
 }
 
+// Fast path. Adds |sample| to cached |histogram_pointer|.
+void HistogramAdd(Histogram* histogram_pointer, int sample) {
+  if (!histogram_pointer)
+    return;
+
+  RtcHistogram* ptr = reinterpret_cast<RtcHistogram*>(histogram_pointer);
+  ptr->Add(sample);
+}
+
 SampleInfo::SampleInfo(const std::string& name,
                        int min,
                        int max,