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

content/browser/renderer_host/render_process_host_impl.cc

Index: content/browser/renderer_host/render_process_host_impl.cc
diff --git a/content/browser/renderer_host/render_process_host_impl.cc b/content/browser/renderer_host/render_process_host_impl.cc
index e52c35499c165848a4fa11cd452f95dcfebe57a9..ed13ae8f9d37eb8958455bf600fd7fc59b9c3af3 100644
--- a/content/browser/renderer_host/render_process_host_impl.cc
+++ b/content/browser/renderer_host/render_process_host_impl.cc
@@ -24,8 +24,12 @@
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/macros.h"
+#include "base/memory/shared_memory.h"
+#include "base/memory/shared_memory_handle.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
+#include "base/metrics/persistent_histogram_allocator.h"
+#include "base/metrics/persistent_memory_allocator.h"
 #include "base/process/process_handle.h"
 #include "base/rand_util.h"
 #include "base/single_thread_task_runner.h"
@@ -1134,6 +1138,11 @@ ServiceRegistry* RenderProcessHostImpl::GetServiceRegistry() {
   return mojo_application_host_->service_registry();
 }
 
+scoped_ptr<base::SharedPersistentMemoryAllocator>
+RenderProcessHostImpl::ExtractMetricsAllocator() {
+  return std::move(metrics_allocator_);
+}
+
 const base::TimeTicks& RenderProcessHostImpl::GetInitTimeForNavigationMetrics()
     const {
   return init_time_;
@@ -2589,6 +2598,26 @@ void RenderProcessHostImpl::OnProcessLaunched() {
 #else
     UpdateProcessPriority();
 #endif
+
+    // Create a persistent memory segment for renderer histograms only if

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

Please make a separate function for this block to encapsulate the logic.

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

Done.

+    // they're active in the browser.
+    if (base::PersistentHistogramAllocator::GetGlobalAllocator()) {
+      DCHECK(!metrics_allocator_);
+      // TODO(bcwhite): Update this with the correct memory size.
+      scoped_ptr<base::SharedMemory> shm(new base::SharedMemory());
+      shm->CreateAndMapAnonymous(2 << 20);  // 2 MiB
+      metrics_allocator_.reset(new base::SharedPersistentMemoryAllocator(
+          std::move(shm), child_process_launcher_->GetProcess().Pid(),
+          "RendererMetrics", /*readonly=*/false));
+
+      if (metrics_allocator_) {
+        base::SharedMemoryHandle shm_handle;
+        metrics_allocator_->shared_memory()->ShareToProcess(
+            child_process_launcher_->GetProcess().Handle(), &shm_handle);
+        Send(new ChildProcessMsg_SetHistogramMemory(
+            shm_handle, metrics_allocator_->shared_memory()->mapped_size()));
+      }
+    }
   }
 
   // NOTE: This needs to be before sending queued messages because