Decommits when coallescing spans instead of committing.

third_party/tcmalloc/page_heap.h

Index: third_party/tcmalloc/page_heap.h
diff --git a/third_party/tcmalloc/page_heap.h b/third_party/tcmalloc/page_heap.h
index 8d9822c6d58d3dc057b4cf4f243a6e280fb6038e..f6a85eee05aa1d4e4fac98ff375c6d7cae703590 100644
--- a/third_party/tcmalloc/page_heap.h
+++ b/third_party/tcmalloc/page_heap.h
@@ -213,6 +213,9 @@ class PageHeap {
   // Commit the span.
   void CommitSpan(Span* span);
 
+  // Decommits the span
+  void DecommitSpan(Span* span);
+
   // Incrementally release some memory to the system.
   // IncrementalScavenge(n) is called whenever n pages are freed.
   void IncrementalScavenge(Length n);