Add ImmutableHeap class consisting of parts

src/vm/object_memory.h

Index: src/vm/object_memory.h
diff --git a/src/vm/object_memory.h b/src/vm/object_memory.h
index dd2187a8da191390f2107b339d7c980eea9ffe01..3d1f8bfc5870688f9615d58e44700002738952d3 100644
--- a/src/vm/object_memory.h
+++ b/src/vm/object_memory.h
@@ -72,6 +72,8 @@ class Chunk {
 // Space is a chain of chunks. It supports allocation and traversal.
 class Space {
  public:
+  static const int kDefaultChunkSize = 128 * KB;
+
   explicit Space(int maximum_initial_size = 0);
 
   ~Space();
@@ -111,6 +113,8 @@ class Space {
   // Adjust the allocation budget based on the current heap size.
   void AdjustAllocationBudget();
 
+  void SetAllocationBudget(int new_budget);
+
   // Tells whether garbage collection is needed.
   bool needs_garbage_collection() { return allocation_budget_ <= 0; }
 
@@ -132,8 +136,6 @@ class Space {
  private:
   friend class NoAllocationFailureScope;
 
-  static const int kDefaultChunkSize = 128 * KB;
-
   uword TryAllocate(int size);
   uword AllocateInNewChunk(int size);