Add the "virtual" keyword on method overrides that are missing it.

chrome/renderer/renderer_histogram_snapshots.h

Index: chrome/renderer/renderer_histogram_snapshots.h
diff --git a/chrome/renderer/renderer_histogram_snapshots.h b/chrome/renderer/renderer_histogram_snapshots.h
index 5155b6f0ee7164fbce8ffd1409af8bc318054110..de3ccc429a80e5115b689acf275da1cbd93299fe 100644
--- a/chrome/renderer/renderer_histogram_snapshots.h
+++ b/chrome/renderer/renderer_histogram_snapshots.h
@@ -37,12 +37,12 @@ class RendererHistogramSnapshots : public HistogramSender {
       renderer_histogram_snapshots_factory_;
 
   // HistogramSender interface (override) methods.
-  void TransmitHistogramDelta(
+  virtual void TransmitHistogramDelta(
       const base::Histogram& histogram,
       const base::Histogram::SampleSet& snapshot);
-  void InconsistencyDetected(int problem);
-  void UniqueInconsistencyDetected(int problem);
-  void SnapshotProblemResolved(int amount);
+  virtual void InconsistencyDetected(int problem);
+  virtual void UniqueInconsistencyDetected(int problem);
+  virtual void SnapshotProblemResolved(int amount);
 
   // Collection of histograms to send to the browser.
   HistogramPickledList pickled_histograms_;