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

content/common/child_process_messages.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Common IPC messages used for child processes.
 // Multiply-included message file, hence no include guard.
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 83 matching lines @@
 // tracked_objects).
 IPC_MESSAGE_CONTROL2(ChildProcessMsg_GetChildProfilerData,
                      int /* sequence_number */,
                      int /* current_profiling_phase */)
 
 // Send to all the child processes to mark the current profiling phase as
 // finished and start a new phase.
 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 */)
 
 // Sent to child processes to tell them to enter or leave background mode.
 IPC_MESSAGE_CONTROL1(ChildProcessMsg_SetProcessBackgrounded,
                      bool /* background */)
 
 // Sends a pipe used by the child process to broker passing of Mojo handles.
 IPC_MESSAGE_CONTROL1(ChildProcessMsg_SetMojoParentPipeHandle,
@@ skipping 93 matching lines @@
 IPC_SYNC_MESSAGE_CONTROL2_1(
     ChildProcessHostMsg_SyncAllocateLockedDiscardableSharedMemory,
     uint32_t /* size */,
     content::DiscardableSharedMemoryId,
     base::SharedMemoryHandle)
 
 // Informs the browser that the child deleted a block of discardable shared
 // memory.
 IPC_MESSAGE_CONTROL1(ChildProcessHostMsg_DeletedDiscardableSharedMemory,
                      content::DiscardableSharedMemoryId)