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

content/child/child_histogram_message_filter.cc

Index: content/child/child_histogram_message_filter.cc
diff --git a/content/child/child_histogram_message_filter.cc b/content/child/child_histogram_message_filter.cc
index f749cf812fb571c502af103b8068c53b71aeb271..9effd8d4d4554aa21c84fbb6b54a723031215b7a 100644
--- a/content/child/child_histogram_message_filter.cc
+++ b/content/child/child_histogram_message_filter.cc
@@ -9,6 +9,7 @@
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/metrics/histogram_delta_serialization.h"
+#include "base/metrics/histogram_persistence.h"
 #include "base/single_thread_task_runner.h"
 #include "content/child/child_process.h"
 #include "content/common/child_process_messages.h"
@@ -35,6 +36,8 @@ bool ChildHistogramMessageFilter::OnMessageReceived(
     const IPC::Message& message) {
   bool handled = true;
   IPC_BEGIN_MESSAGE_MAP(ChildHistogramMessageFilter, message)
+    IPC_MESSAGE_HANDLER(ChildProcessMsg_SetHistogramMemory,
+                        OnSetHistogramMemory)
     IPC_MESSAGE_HANDLER(ChildProcessMsg_GetChildHistogramData,
                         OnGetChildHistogramData)
     IPC_MESSAGE_UNHANDLED(handled = false)
@@ -42,6 +45,20 @@ bool ChildHistogramMessageFilter::OnMessageReceived(
   return handled;
 }
 
+void ChildHistogramMessageFilter::OnSetHistogramMemory(
+    const base::SharedMemoryHandle& mem_handle,
+    int mem_size) {
+  scoped_ptr<base::PersistentMemoryAllocator> allocator(
+      base::CreateSharedPersistentHistogramMemoryAllocator(
+          mem_size, mem_handle, false));
+
+  if (allocator) {
+    base::SetPersistentHistogramMemoryAllocator(allocator.release());

[Alexei Svitkine (slow), 2016/03/24 17:29:53]

What happens if there's already an allocator set? Will it get lost? If so, adds
some checks against this since it seems we don't want that to happen.

[bcwhite, 2016/03/29 11:53:27]

It dies with a CHECK().

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

That's fair. Maybe add a comment mentioning that the expectation is that this
message is only received once and that it will trigger a CHECK if it gets called
multiple times.

[bcwhite, 2016/03/30 21:25:55]

Done.

+    base::GetPersistentHistogramMemoryAllocator()->CreateTrackingHistograms(
+        base::GetPersistentHistogramMemoryAllocator()->Name());
+  }
+}
+
 void ChildHistogramMessageFilter::SendHistograms(int sequence_number) {
   io_task_runner_->PostTask(
       FROM_HERE, base::Bind(&ChildHistogramMessageFilter::UploadAllHistograms,