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

content/public/browser/render_process_host.h

Index: content/public/browser/render_process_host.h
diff --git a/content/public/browser/render_process_host.h b/content/public/browser/render_process_host.h
index c9c13183c9e1bb782912f58906faf1c65be88573..1db47c58bf583a50e7a9fe0b4832b6420f3d68c0 100644
--- a/content/public/browser/render_process_host.h
+++ b/content/public/browser/render_process_host.h
@@ -22,6 +22,7 @@
 class GURL;
 
 namespace base {
+class SharedPersistentMemoryAllocator;
 class TimeDelta;
 }
 
@@ -254,6 +255,14 @@ class CONTENT_EXPORT RenderProcessHost : public IPC::Sender,
   // Returns the ServiceRegistry for this process.
   virtual ServiceRegistry* GetServiceRegistry() = 0;
 
+  // Extracts any persistent-memory-allocator used for renderer metrics.
+  // Ownership is passed to the caller. To support sharing of histogram data
+  // between the Renderer and the Browser, the allocator is created when the
+  // process is created and later retrieved by the SubprocessMetricsProvider
+  // for management.
+  virtual scoped_ptr<base::SharedPersistentMemoryAllocator>
+  TakeMetricsAllocator() = 0;
+
   // PlzNavigate
   // Returns the time the first call to Init completed successfully (after a new
   // renderer process was created); further calls to Init won't change this