Display histograms from subprocesses in chrome://histograms

base/metrics/histogram_base.h

Index: base/metrics/histogram_base.h
diff --git a/base/metrics/histogram_base.h b/base/metrics/histogram_base.h
index 58a9ab2edc093393fdd7177c3f99f47b543b41e4..1dbe679bd01d1f76cc06beff04839a550db5e6f5 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -211,9 +211,13 @@ class BASE_EXPORT HistogramBase {
   // read-only memory.
   virtual std::unique_ptr<HistogramSamples> SnapshotFinalDelta() const = 0;
 
-  // The following methods provide graphical histogram displays.
-  virtual void WriteHTMLGraph(std::string* output) const = 0;
-  virtual void WriteAscii(std::string* output) const = 0;
+  // The following methods provide graphical histogram displays. The snapshot
+  // must be for this histogram or the results are undefined. If the passed
+  // |snapshot| is null, one will be taken of the internal data.
+  virtual void WriteHTMLGraph(const HistogramSamples* snapshot,
+                              std::string* output) const = 0;
+  virtual void WriteAscii(const HistogramSamples* snapshot,
+                          std::string* output) const = 0;
 
   // Produce a JSON representation of the histogram. This is implemented with
   // the help of GetParameters and GetCountAndBucketData; overwrite them to