Before write-protecting a PageSpace, return bump allocation block to freelist.

runtime/vm/pages.h

Index: runtime/vm/pages.h
===================================================================
--- runtime/vm/pages.h	(revision 45122)
+++ runtime/vm/pages.h	(working copy)
@@ -368,6 +368,8 @@
                                     bool is_locked);
   // Makes bump block walkable; do not call concurrently with mutator.
   void MakeIterable() const;
+  // Return any bump allocation block to the freelist.
+  void AbandonBumpAllocation();
   HeapPage* AllocatePage(HeapPage::PageType type);
   void FreePage(HeapPage* page, HeapPage* previous_page);
   HeapPage* AllocateLargePage(intptr_t size, HeapPage::PageType type);