Lock tcmalloc structures in a pthread_atfork hook.

third_party/tcmalloc/chromium/src/libc_override_osx.h

Index: third_party/tcmalloc/chromium/src/libc_override_osx.h
diff --git a/third_party/tcmalloc/chromium/src/libc_override_osx.h b/third_party/tcmalloc/chromium/src/libc_override_osx.h
index 78a0ef2f95974b4d02e75dceaa716f75592d1e0a..04360ce500a11fca47c893e22b64d7acca9759fd 100644
--- a/third_party/tcmalloc/chromium/src/libc_override_osx.h
+++ b/third_party/tcmalloc/chromium/src/libc_override_osx.h
@@ -84,6 +84,11 @@
 #include <AvailabilityMacros.h>
 #include <malloc/malloc.h>
 
+namespace tcmalloc {
+  void CentralCacheLockAll();
+  void CentralCacheUnlockAll();
+}
+
 // from AvailabilityMacros.h
 #if defined(MAC_OS_X_VERSION_10_6) && \
     MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_6
@@ -168,11 +173,11 @@ void mi_log(malloc_zone_t *zone, void *address) {
 }
 
 void mi_force_lock(malloc_zone_t *zone) {
-  // Hopefully unneeded by us!
+  tcmalloc::CentralCacheLockAll();
 }
 
 void mi_force_unlock(malloc_zone_t *zone) {
-  // Hopefully unneeded by us!
+  tcmalloc::CentralCacheUnlockAll();
 }
 
 void mi_statistics(malloc_zone_t *zone, malloc_statistics_t *stats) {