Support "shared" histograms between processes.

base/metrics/histogram_base.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_METRICS_HISTOGRAM_BASE_H_
 #define BASE_METRICS_HISTOGRAM_BASE_H_
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
 
 #include <string>
 #include <vector>
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string_piece.h"
 #include "base/time/time.h"
 
 namespace base {
 
+class BucketRanges;
 class DictionaryValue;
 class HistogramBase;
 class HistogramSamples;
 class ListValue;
 class Pickle;
 class PickleIterator;
 
 ////////////////////////////////////////////////////////////////////////////////
 // These enums are used to facilitate deserialization of histograms from other
 // processes into the browser. If you create another class that inherits from
@@ skipping 40 matching lines @@
     // aggregation should not take place (as we would be aggregating back into
     // the source histogram!).
     kIPCSerializationSourceFlag = 0x10,
 
     // Indicates that a callback exists for when a new sample is recorded on
     // this histogram. We store this as a flag with the histogram since
     // histograms can be in performance critical code, and this allows us
     // to shortcut looking up the callback if it doesn't exist.
     kCallbackExists = 0x20,
 
+    // Indicates that the histogram is held in "persistent" memory and may
+    // be accessible between processes. This is only possible if such a
+    // memory segment has been created/attached, used to create a Persistent-
+    // MemoryAllocator, and that loaded into the Histogram module before this
+    // histogram is created.
+    kIsPersistent = 0x40,
+
     // Only for Histogram and its sub classes: fancy bucket-naming support.
     kHexRangePrintingFlag = 0x8000,
   };
 
   // Histogram data inconsistency types.
   enum Inconsistency {
     NO_INCONSISTENCIES = 0x0,
     RANGE_CHECKSUM_ERROR = 0x1,
     BUCKET_ORDER_ERROR = 0x2,
     COUNT_HIGH_ERROR = 0x4,
@@ skipping 101 matching lines @@
  private:
   const std::string histogram_name_;
   AtomicCount flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_