Align double array backing store during compaction and mark-sweep promotion.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index d2d69618934f3eb7799423a1ecec2941ce750bd3..dbd49bf8480a24fc2a80a095395f500dfc54917c 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1422,6 +1422,16 @@ class Heap {
   // Write barrier support for address[start : start + len[ = o.
   INLINE(void RecordWrites(Address address, int start, int len));
 
+  // Return true if object on heap have to be double aligned.
+  INLINE(static bool MustBeDoubleAligned(HeapObject* object));
+
+  // Ensure object is 8-byte aligned after raw allocation.
+  // Return aligned address. One word in allocated space is prefilled to
+  // provide padding, so value of the size argument is amount of allocated
+  // memory for the object that must be one word larger than actual size of
+  // the object.
+  INLINE(HeapObject* EnsureDoubleAligned(HeapObject* object, int size));
+
   enum HeapState { NOT_IN_GC, SCAVENGE, MARK_COMPACT };
   inline HeapState gc_state() { return gc_state_; }