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

third_party/tcmalloc/chromium/src/base/stl_allocator.h

Index: third_party/tcmalloc/chromium/src/base/stl_allocator.h
===================================================================
--- third_party/tcmalloc/chromium/src/base/stl_allocator.h	(revision 88058)
+++ third_party/tcmalloc/chromium/src/base/stl_allocator.h	(working copy)
@@ -59,7 +59,7 @@
 class STL_Allocator {
  public:
   typedef size_t     size_type;
-  typedef ptrdiff_t  difference_type;
+  typedef std::ptrdiff_t  difference_type;
   typedef T*         pointer;
   typedef const T*   const_pointer;
   typedef T&         reference;