Adapt upstream patches fixing potential deadlock in tcmalloc after fork.

third_party/tcmalloc/chromium/src/central_freelist.h

Index: third_party/tcmalloc/chromium/src/central_freelist.h
diff --git a/third_party/tcmalloc/chromium/src/central_freelist.h b/third_party/tcmalloc/chromium/src/central_freelist.h
index 4fd5799de9ab53a6458942edaa056b7b56e174b5..e547f15aff0d5790b203c47e161526ed23942f3d 100644
--- a/third_party/tcmalloc/chromium/src/central_freelist.h
+++ b/third_party/tcmalloc/chromium/src/central_freelist.h
@@ -79,6 +79,16 @@ class CentralFreeList {
   // page full of 5-byte objects would have 2 bytes memory overhead).
   size_t OverheadBytes();
 
+  // Lock/Unlock the internal SpinLock. Used on the pthread_atfork call
+  // to set the lock in a consistent state before the fork.
+  void Lock() {
+    lock_.Lock();
+  }
+
+  void Unlock() {
+    lock_.Unlock();
+  }
+
  private:
   // TransferCache is used to cache transfers of
   // sizemap.num_objects_to_move(size_class) back and forth between