NOT YET READY: UMA recording the kind of target frame when extensions pierce browsing instance.

base/test/histogram_tester.h

Index: base/test/histogram_tester.h
diff --git a/base/test/histogram_tester.h b/base/test/histogram_tester.h
index 1bd024c69f9f324538fdedb19a75c2aa646d3a68..acaae2cf9e6ee1183e579cceaf585133b229dec2 100644
--- a/base/test/histogram_tester.h
+++ b/base/test/histogram_tester.h
@@ -25,6 +25,10 @@ class HistogramSamples;
 // HistogramTester provides a simple interface for examining histograms, UMA
 // or otherwise. Tests can use this interface to verify that histogram data is
 // getting logged as intended.
+//
+// Note: When using this class from a browser test, one might have to call
+// SubprocessMetricsProvider::MergeHistogramDeltasForTesting() to sync the
+// histogram data between the renderer and browser processes.
 class HistogramTester {
  public:
   using CountsMap = std::map<std::string, HistogramBase::Count>;