Add implementation in metrics.h that uses atomic pointer.
Update test implementation (test/histograms.h) to be more similar a real implementation (where histogram get functions return a Histogram pointer). Add check that the name of a histogram does not change.
BUG=webrtc:5283
Review URL: https://codereview.webrtc.org/1528403003
Cr-Commit-Position: refs/heads/master@{#11161}
diff --git a/webrtc/test/histogram.h b/webrtc/test/histogram.h
index 44ce32b..3c8e743 100644
--- a/webrtc/test/histogram.h
+++ b/webrtc/test/histogram.h
@@ -23,7 +23,7 @@
// Returns the number of added samples to a histogram.
int NumHistogramSamples(const std::string& name);
-// Removes all histograms.
+// Removes all histogram samples.
void ClearHistograms();
} // namespace test