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

content/child/child_histogram_message_filter.h

Index: content/child/child_histogram_message_filter.h
diff --git a/content/child/child_histogram_message_filter.h b/content/child/child_histogram_message_filter.h
index e035e3ad7056a51e07e0c1a32964f64eb9bd4252..d9b214fba5c9a4b49c789d4ff8f5d0a9f79dfbb8 100644
--- a/content/child/child_histogram_message_filter.h
+++ b/content/child/child_histogram_message_filter.h
@@ -10,6 +10,7 @@
 
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/memory/shared_memory.h"
 #include "ipc/message_filter.h"
 
 namespace base {
@@ -36,6 +37,8 @@ class ChildHistogramMessageFilter : public IPC::MessageFilter {
   ~ChildHistogramMessageFilter() override;
 
   // Message handlers.
+  virtual void OnSetHistogramMemory(const base::SharedMemoryHandle& mem_handle,

[Alexei Svitkine (slow), 2016/03/29 17:51:24]

Does this need to be virtual? If it doesn't probably the one below doesn't
either.

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

Done.  It looks like it was done that way to match the other OnFoo handlers that
are inherited virtual from IPC::MessageFilter.

+                                    int mem_size);
   virtual void OnGetChildHistogramData(int sequence_number);
 
   // Extract snapshot data and then send it off the the Browser process.