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..4918e598a427db01428a365753797502f36877e5 100644
--- a/content/common/child_process_messages.h
+++ b/content/common/child_process_messages.h
@@ -101,8 +101,16 @@ 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) will return any histograms created before this

[grt (UTC plus 2), 2016/04/01 14:22:49]

GetChildNonPersistentHistogramData

[bcwhite, 2016/04/01 16:50:26]

Done.

+// message is received but not any histograms created afterward.
+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,
+IPC_MESSAGE_CONTROL1(ChildProcessMsg_GetChildNonPersistentHistogramData,
                      int /* sequence_number */)
 
 // Sent to child processes to tell them to enter or leave background mode.