Add a comment to the allocator shim.

base/allocator/allocator_shim.cc

 // Copyright 2016 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.
 
 #include "base/allocator/allocator_shim.h"
 
 #include <errno.h>
 
 #include <new>
 
@@ skipping 229 matching lines @@
   return ShimMemalign(GetCachedPageSize(), size, context);
 }
 
 void* ShimPvalloc(size_t size) {
   // pvalloc(0) should allocate one page, according to its man page.
   if (size == 0) {
     size = GetCachedPageSize();
   } 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);
 }
 
 void ShimFree(void* address, void* context) {
   const allocator::AllocatorDispatch* const chain_head = GetChainHead();
   return chain_head->free_function(chain_head, address, context);
 }
 
 size_t ShimGetSizeEstimate(const void* address, void* context) {
   const allocator::AllocatorDispatch* const chain_head = GetChainHead();
@@ skipping 73 matching lines @@
 // Cross-checks.
 
 #if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
 #error The allocator shim should not be compiled when building for memory tools.
 #endif
 
 #if (defined(__GNUC__) && defined(__EXCEPTIONS)) || \
     (defined(_HAS_EXCEPTIONS) && _HAS_EXCEPTIONS)
 #error This code cannot be used when exceptions are turned on.
 #endif