Add a clarifying comment to the memory layout navigation. (Inspired by Coverity CID 100022)

courgette/memory_allocator.cc

 // Copyright (c) 2011 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 "courgette/memory_allocator.h"
 
 #include <map>
 
 #include "base/file_util.h"
 #include "base/stringprintf.h"
@@ skipping 117 matching lines @@
   }
 
   TempMapping** write = reinterpret_cast<TempMapping**>(mapping_.view());
   write[0] = this;
 
   return true;
 }
 
 void* TempMapping::memory() const {
   uint8* mem = reinterpret_cast<uint8*>(mapping_.view());
+  // The 'this' pointer is written at the start of mapping_.view(), so
+  // go past it. (See Initialize()).
   if (mem)
     mem += sizeof(this);
   DCHECK(mem);
   return mem;
 }
 
 bool TempMapping::valid() const {
   return mapping_.valid();
 }
 
 // static
 TempMapping* TempMapping::GetMappingFromPtr(void* mem) {
   TempMapping* ret = NULL;
   if (mem) {
     ret = reinterpret_cast<TempMapping**>(mem)[-1];
   }
   DCHECK(ret);
   return ret;
 }
 
 }  // namespace courgette
 
 #endif  // OS_WIN