Add a comment to the allocator shim.

base/allocator/allocator_shim.cc

Index: base/allocator/allocator_shim.cc
diff --git a/base/allocator/allocator_shim.cc b/base/allocator/allocator_shim.cc
index 4de1a8b3b9e368730337a8a1378772ebae38d33a..48b13501fe443ddc2062a1fab5e14a43a3a19649 100644
--- a/base/allocator/allocator_shim.cc
+++ b/base/allocator/allocator_shim.cc
@@ -247,6 +247,8 @@ void* ShimPvalloc(size_t size) {
   } else {
     size = (size + GetCachedPageSize() - 1) & ~(GetCachedPageSize() - 1);
   }
+  // The third argument is nullptr because pvalloc is glibc only and does not
+  // exist on OSX/BSD systems.
   return ShimMemalign(GetCachedPageSize(), size, nullptr);
 }