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 62d65fca999490f31592be64bad59f5327b45c8c..4abeb5d79873f1eba0e4240e588992f7087f81b1 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;
 }
 
@@ -258,6 +259,14 @@ 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 th ecaller. To support sharing of histogram data

[Alexei Svitkine (slow), 2016/03/29 17:51:25]

Nit: "th ecaller"

[bcwhite, 2016/03/30 21:25:56]

Done.

+  // 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>
+  ExtractMetricsAllocator() = 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