Improve UMA tests; improve no-op reconcilor; new reconcilor test.

chrome/test/base/uma_histogram_helper.h

Index: chrome/test/base/uma_histogram_helper.h
diff --git a/chrome/test/base/uma_histogram_helper.h b/chrome/test/base/uma_histogram_helper.h
index 76e841fcfd3651266eb709273d8a9b20d44ff400..2feb4d534e0972b2836aa13ea54e474cfee4a976 100644
--- a/chrome/test/base/uma_histogram_helper.h
+++ b/chrome/test/base/uma_histogram_helper.h
@@ -5,6 +5,7 @@
 #ifndef CHROME_TEST_BASE_UMA_HISTOGRAM_HELPER_H_
 #define CHROME_TEST_BASE_UMA_HISTOGRAM_HELPER_H_
 
+#include "base/memory/linked_ptr.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
@@ -14,25 +15,40 @@
 // intended.
 class UMAHistogramHelper {
  public:
+  // UMAHistogramHelper should be created before the execution of the test case.
   UMAHistogramHelper();
 
+  ~UMAHistogramHelper();
+
+  // Parameters should be string literals of all histograms to snapshot.
+  // Call this before executing the test code. This method can be called
+  // multiple times. The existing snapshots are preserved, except when one of
+  // the |histogram_names| was previously passed as a parameter, then a new
+  // snapshot will replace the existing one.
+  void PrepareSnapshot(const char* const histogram_names[],
+                       size_t num_histograms);
+
   // Each child process may have its own histogram data, make sure this data
   // gets accumulated into the browser process before we examine the histograms.
   void Fetch();
 
   // We know the exact number of samples in a bucket, and that no other bucket
-  // should have samples.
+  // should have samples. If |PrepareSnapshot| was called for the histogram
+  // named |name| then the |expected_count| is the diff from the snapshot.
   void ExpectUniqueSample(const std::string& name,
                           base::HistogramBase::Sample sample,
                           base::HistogramBase::Count expected_count);
 
   // We know the exact number of samples in a bucket, but other buckets may
-  // have samples as well.
+  // have samples as well. If |PrepareSnapshot| was called for histogram named
+  // |name| then the |expected_count| is the diff from the snapshot.
   void ExpectBucketCount(const std::string& name,
                           base::HistogramBase::Sample sample,
                           base::HistogramBase::Count expected_count);
 
-  // We don't know the values of the samples, but we know how many there are.
+  // We don't know the values of the samples, but we know how many there are. If
+  // |PrepareSnapshot| was called for |name| histogram, then the
+  // |count| is the diff from the snapshot.
   void ExpectTotalCount(const std::string& name,
                         base::HistogramBase::Count count);
 
@@ -49,6 +65,10 @@ class UMAHistogramHelper {
                        base::HistogramSamples& samples);
 
   DISALLOW_COPY_AND_ASSIGN(UMAHistogramHelper);
+
+  // The map from histogram names to their snapshots
+  std::map<std::string, linked_ptr<base::HistogramSamples> >
+      histogram_snapshots;
 };
 
 #endif  // CHROME_TEST_BASE_UMA_HISTOGRAM_HELPER_H_