Fix another FastMalloc call site with potential integer overflow.

third_party/WebKit/Source/platform/wtf/allocator/Partitions.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 153 matching lines @@
 using base::SetSystemPagesAccessible;
 using base::SetSystemPagesInaccessible;
 
 using base::PageAccessible;
 using base::PageInaccessible;
 using base::PartitionStatsDumper;
 using base::PartitionMemoryStats;
 using base::PartitionBucketMemoryStats;
 using base::PartitionAllocHooks;
 
+using CheckedSizeT = base::CheckedNumeric<size_t>;
+
 }  // namespace WTF
 
 #endif  // Partitions_h