Create and pass shared-histogram-allocator from browser to renderer.

base/metrics/histogram_persistence.h

Index: base/metrics/histogram_persistence.h
diff --git a/base/metrics/histogram_persistence.h b/base/metrics/histogram_persistence.h
index 95f48784c9859a3a0003092ca37f258ee047a1b9..0bd8033f53c9b1f4bc55118fb1eec9915bace90f 100644
--- a/base/metrics/histogram_persistence.h
+++ b/base/metrics/histogram_persistence.h
@@ -8,11 +8,15 @@
 #include "base/base_export.h"
 #include "base/feature_list.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/memory/shared_memory.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/persistent_memory_allocator.h"
+#include "base/strings/string_piece.h"
 
 namespace base {
 
+class SharedPersistentMemoryAllocator;
+
 // Feature definition for enabling histogram persistence.
 BASE_EXPORT extern const Feature kPersistentHistogramsFeature;
 
@@ -39,6 +43,21 @@ BASE_EXPORT PersistentMemoryAllocator* GetPersistentHistogramMemoryAllocator();
 BASE_EXPORT PersistentMemoryAllocator*
 ReleasePersistentHistogramMemoryAllocatorForTesting();
 
+// Create a sharable memory segment with an attached persistent memory
+// allocator suitable for storing histograms. This segment can be shared
+// with other processes via an IPC message and used to create the same
+// on the other side.

[Alexei Svitkine (slow), 2016/03/24 17:29:53]

Please document the params better. What's id? What's the difference between the
two functions? I suggest having separate comments for them.

+BASE_EXPORT SharedPersistentMemoryAllocator*
+CreateSharedPersistentHistogramMemoryAllocator(
+    size_t size,
+    StringPiece name,
+    uint64_t id);
+BASE_EXPORT SharedPersistentMemoryAllocator*
+CreateSharedPersistentHistogramMemoryAllocator(

[Alexei Svitkine (slow), 2016/03/24 17:29:52]

Overloads are generally discouraged. Please use different names for the two
functions.

+    size_t size,
+    const SharedMemoryHandle& handle,
+    bool readonly);
+
 // Recreate a Histogram from data held in persistent memory. Though this
 // object will be local to the current process, the sample data will be
 // shared with all other threads referencing it. This method takes a |ref|