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

content/common/child_process_messages.h

Index: content/common/child_process_messages.h
diff --git a/content/common/child_process_messages.h b/content/common/child_process_messages.h
index 9476f1a5163504c4ce7a4eff46f3016a48cf6055..6afe10ffb200dd7afa4ce63debe6f51be643fd34 100644
--- a/content/common/child_process_messages.h
+++ b/content/common/child_process_messages.h
@@ -101,6 +101,14 @@ IPC_MESSAGE_CONTROL2(ChildProcessMsg_GetChildProfilerData,
 IPC_MESSAGE_CONTROL1(ChildProcessMsg_ProfilingPhaseCompleted,
                      int /* profiling_phase */)
 
+// Sent to set the shared memory buffer to be used for storing histograms that
+// are to be reported by the browser process to UMA. The following message
+// (GetChildHistogramData) won't return any histograms created after this is
+// processed.

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

Please add a note about the histograms that are created before this message is
received - i.e. they will still be retrieved via GetChildHistogramData.

Maybe we should change the name of the other function to something like
GetChildNonPersistentHistogramData()?

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

Done.
Done.

+IPC_MESSAGE_CONTROL2(ChildProcessMsg_SetHistogramMemory,
+                     base::SharedMemoryHandle /* shm_handle */,
+                     int /* shm_size */)
+
 // Send to all the child processes to send back histogram data.
 IPC_MESSAGE_CONTROL1(ChildProcessMsg_GetChildHistogramData,
                      int /* sequence_number */)