Merge multiple histogram snapshots into single one for reporting.

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 40c2caedc313a815c6400b9893feb093c1a6d676..6817629bf8c080dd9a3fe259d96ff586c7ccebc2 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -94,14 +94,18 @@ class BASE_EXPORT HistogramBase {
   };
 
   // Histogram data inconsistency types.
-  enum Inconsistency {
+  enum Inconsistency : uint32_t {
     NO_INCONSISTENCIES = 0x0,
     RANGE_CHECKSUM_ERROR = 0x1,
     BUCKET_ORDER_ERROR = 0x2,
     COUNT_HIGH_ERROR = 0x4,
     COUNT_LOW_ERROR = 0x8,
 
-    NEVER_EXCEEDED_VALUE = 0x10
+    NEVER_EXCEEDED_VALUE = 0x10,
+
+    // This value is used only in HistogramSnapshotManager for marking
+    // internally when new inconsistencies are found.
+    NEW_INCONSISTENCY_FOUND = 0x8000000
   };
 
   explicit HistogramBase(const std::string& name);
@@ -154,12 +158,17 @@ class BASE_EXPORT HistogramBase {
 
   // Try to find out data corruption from histogram and the samples.
   // The returned value is a combination of Inconsistency enum.
-  virtual int FindCorruption(const HistogramSamples& samples) const;
+  virtual uint32_t FindCorruption(const HistogramSamples& samples) const;
 
   // Snapshot the current complete set of sample data.
   // Override with atomic/locked snapshot if needed.
   virtual scoped_ptr<HistogramSamples> SnapshotSamples() const = 0;
 
+  // Calculate the change (delta) in histogram counts since the previous call
+  // to this method. Each successive call will return only those counts
+  // changed since the last call.
+  virtual scoped_ptr<HistogramSamples> SnapshotDelta() = 0;
+
   // The following methods provide graphical histogram displays.
   virtual void WriteHTMLGraph(std::string* output) const = 0;
   virtual void WriteAscii(std::string* output) const = 0;