Support "shared" histograms between processes.

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 f45cc9c106e323e83791265a57cbefaa3352011b..ffde2cfb39b8b8eba81d4dd5b2827028cc113752 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -19,6 +19,7 @@
 
 namespace base {
 
+class BucketRanges;
 class DictionaryValue;
 class HistogramBase;
 class HistogramSamples;
@@ -79,6 +80,13 @@ class BASE_EXPORT HistogramBase {
     // 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,
   };