Replaced ptrdiff_t with std::ptrdiff_t in all files to enable build with gcc-4.6.0.

third_party/tcmalloc/chromium/src/system-alloc.cc

Index: third_party/tcmalloc/chromium/src/system-alloc.cc
===================================================================
--- third_party/tcmalloc/chromium/src/system-alloc.cc	(revision 88058)
+++ third_party/tcmalloc/chromium/src/system-alloc.cc	(working copy)
@@ -166,7 +166,7 @@
 
   // sbrk will release memory if passed a negative number, so we do
   // a strict check here
-  if (static_cast<ptrdiff_t>(size + alignment) < 0) return NULL;
+  if (static_cast<std::ptrdiff_t>(size + alignment) < 0) return NULL;
 
   // This doesn't overflow because TCMalloc_SystemAlloc has already
   // tested for overflow at the alignment boundary.