Adjust memory sizes of persistent histogram allocations.

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 43b5cf5cc0500c4870d8d85e3beee7fe67f9d1dc..001d6d32eda46eb56685ee7ec2466093d591e436 100644
--- a/content/browser/renderer_host/render_process_host_impl.cc
+++ b/content/browser/renderer_host/render_process_host_impl.cc
@@ -2597,7 +2597,10 @@ void RenderProcessHostImpl::CreateSharedRendererHistogramAllocator() {
   // method will get called a second time meaning that a metrics-allocator
   // already exists. Don't recreate it.
   if (!metrics_allocator_) {
-    // TODO(bcwhite): Update this with the correct memory size.
+    // Create persistent/shared memory and allow histograms to be stored in
+    // it. Memory that is not actualy used won't be physically mapped by the
+    // system. RendererMetrics usage, as reported in UMA, peaked around 0.7MiB
+    // as of 2016-12-20.
     std::unique_ptr<base::SharedMemory> shm(new base::SharedMemory());
     if (!shm->CreateAndMapAnonymous(2 << 20))  // 2 MiB
       return;