Update the tcmalloc vendor branch to r144 (gperftools 2.0).

third_party/tcmalloc/vendor/src/libc_override_osx.h

Index: third_party/tcmalloc/vendor/src/libc_override_osx.h
diff --git a/third_party/tcmalloc/vendor/src/libc_override_osx.h b/third_party/tcmalloc/vendor/src/libc_override_osx.h
index 0ccf9a31d2d392d96813b3eba6da0c0fc8eb30e8..78a0ef2f95974b4d02e75dceaa716f75592d1e0a 100644
--- a/third_party/tcmalloc/vendor/src/libc_override_osx.h
+++ b/third_party/tcmalloc/vendor/src/libc_override_osx.h
@@ -75,7 +75,7 @@
 #ifdef HAVE_FEATURES_H
 #include <features.h>
 #endif
-#include <google/tcmalloc.h>
+#include <gperftools/tcmalloc.h>
 
 #if !defined(__APPLE__)
 # error libc_override_glibc-osx.h is for OS X distributions only.
@@ -84,6 +84,19 @@
 #include <AvailabilityMacros.h>
 #include <malloc/malloc.h>
 
+// from AvailabilityMacros.h
+#if defined(MAC_OS_X_VERSION_10_6) && \
+    MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_6
+extern "C" {
+  // This function is only available on 10.6 (and later) but the
+  // LibSystem headers do not use AvailabilityMacros.h to handle weak
+  // importing automatically.  This prototype is a copy of the one in
+  // <malloc/malloc.h> with the WEAK_IMPORT_ATTRBIUTE added.
+  extern malloc_zone_t *malloc_default_purgeable_zone(void)
+      WEAK_IMPORT_ATTRIBUTE;
+}
+#endif
+
 // We need to provide wrappers around all the libc functions.
 namespace {
 size_t mz_size(malloc_zone_t* zone, const void* ptr) {
@@ -235,8 +248,13 @@ static void ReplaceSystemAlloc() {
   // doing tiny and small allocs.  Sadly, it assumes that the default
   // zone is the szone implementation from OS X and will crash if it
   // isn't.  By creating the zone now, this will be true and changing
-  // the default zone won't cause a problem.  (OS X 10.6 and higher.)
-  malloc_default_purgeable_zone();
+  // the default zone won't cause a problem.  This only needs to
+  // happen when actually running on OS X 10.6 and higher (note the
+  // ifdef above only checks if we were *compiled* with 10.6 or
+  // higher; at runtime we have to check if this symbol is defined.)
+  if (malloc_default_purgeable_zone) {
+    malloc_default_purgeable_zone();
+  }
 #endif
 
   // Register the tcmalloc zone. At this point, it will not be the