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}

webrtc/system_wrappers/include/metrics_default.h

@@ -18,6 +18,10 @@
 namespace webrtc {
 namespace metrics {
 
+// This class does not actually exist. It is casted to an implementation defined
+// pointer inside the functions.
+class Histogram;
+
 struct SampleInfo {
   SampleInfo(const std::string& name, int min, int max, size_t bucket_count);
   ~SampleInfo();
@@ -51,6 +55,9 @@ int NumSamples(const std::string& name);
 // Returns the minimum sample value (or -1 if the histogram has no samples).
 int MinSample(const std::string& name);
 
+// Function for adding a |sample| to a histogram without checkking the name.
+void HistogramAdd(Histogram* histogram_pointer, int sample);
+
 }  // namespace metrics
 }  // namespace webrtc