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..db0ac7227896e7da774d8d778b575a0ec07ae326 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 {
@@ -36,9 +37,11 @@ class ChildHistogramMessageFilter : public IPC::MessageFilter {
   ~ChildHistogramMessageFilter() override;
 
   // Message handlers.
-  virtual void OnGetChildHistogramData(int sequence_number);
+  void OnSetHistogramMemory(const base::SharedMemoryHandle& memory_handle,
+                            int memory_size);
+  void OnGetChildHistogramData(int sequence_number);
 
-  // Extract snapshot data and then send it off the the Browser process.
+  // Extract snapshot data and then send it off to the Browser process.
   // Send only a delta to what we have already sent.
   void UploadAllHistograms(int sequence_number);