Enable tcmalloc validation by default in debug builds

third_party/tcmalloc/chromium/src/tcmalloc.cc

 // Copyright (c) 2005, 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
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 // allocation size, which is known in the code above, but then is recalculated
 // below.  Another potential optimization would be careful manual inlining of
 // code, but I *think* that the compile will probably do this for me, and I've
 // been careful to avoid aliasing issues that might make a compiler back-off.
 
 // Evolution includes experimenting with different marks, to minimize the chance
 // that a mark would be misunderstood (missed corruption).  The marks are meant
 // to be hashed encoding of the location, so that they can't be copied over a
 // different region (by accident) without being detected (most of the time).
 
-// Uncomment the following define to turn on all the TCMalloc checking.
-// It will cost abotu 2% in performance, but it will catch double frees (most of
+// Enable the following define to turn on all the TCMalloc checking.
+// It will cost about 2% in performance, but it will catch double frees (most of
 // the time), and will often catch allocated-buffer overrun errors.  This
 // validation is only active when TCMalloc is used as the allocator.
-// #define TCMALLOC_VALIDATION
+#ifndef NDEBUG
+#define TCMALLOC_VALIDATION
+#endif
 
 #if !defined(TCMALLOC_VALIDATION)
 
 static size_t ExcludeSpaceForMark(size_t size) { return size; }
 static void AddRoomForMark(size_t* size) {}
 static void ExcludeMarkFromSize(size_t* new_size) {}
 static void MarkAllocatedRegion(void* ptr) {}
 static void ValidateAllocatedRegion(void* ptr, size_t cl) {}
 
 #else  // TCMALLOC_VALIDATION
@@ skipping 117 matching lines @@
   *mark = ~allocated_mark;  //  Distinctively not allocated.
 }
 
 static void MarkAllocatedRegion(void* ptr) {
   if (ptr == NULL) return;
   MarkType* mark = GetMarkLocation(ptr);
   *mark = GetMarkValue(ptr, mark);
 }
 
 #endif  // TCMALLOC_VALIDATION