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 4fa07c64efba5c5b8d565837e22e325b863ae9d7..d8bd0f44edcce1f9f3b26d049c56c54f8bf953b8 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -21,6 +21,7 @@
 
 namespace base {
 
+class BucketRanges;
 class DictionaryValue;
 class HistogramBase;
 class HistogramSamples;
@@ -81,6 +82,13 @@ 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,
   };