Make object accessors more const-correct.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index 96a1a90427f3a5cd8cce00f629d9bd596993890c..fcf7a442011db54071a52ce40d26eec0a857dd3b 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -283,6 +283,10 @@ class MemoryChunk {
   static MemoryChunk* FromAddress(Address a) {
     return reinterpret_cast<MemoryChunk*>(OffsetFrom(a) & ~kAlignmentMask);
   }
+  static const MemoryChunk* FromAddress(const byte* a) {
+    return reinterpret_cast<const MemoryChunk*>(
+        OffsetFrom(a) & ~kAlignmentMask);
+  }
 
   // Only works for addresses in pointer spaces, not data or code spaces.
   static inline MemoryChunk* FromAnyPointerAddress(Heap* heap, Address addr);
@@ -627,7 +631,7 @@ class MemoryChunk {
   void InsertAfter(MemoryChunk* other);
   void Unlink();
 
-  inline Heap* heap() { return heap_; }
+  inline Heap* heap() const { return heap_; }
 
   static const int kFlagsOffset = kPointerSize;