Move double alignment logic into memory allocator.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 71ee26e2cecc76f9784c8790947db2a4c226733f..3e6d5ab39816cc3cf124e90742499ed85a8aabb9 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -714,6 +714,11 @@ class Heap {
   MUST_USE_RESULT AllocationResult
       CopyJSObject(JSObject* source, AllocationSite* site = NULL);
 
+  // This method assumes overallocation of one word. It will store a filler
+  // before the object if the given object is not double aligned, otherwise
+  // it will place the filler after the object.
+  MUST_USE_RESULT HeapObject* EnsureDoubleAligned(HeapObject* object, int size);
+
   // Clear the Instanceof cache (used when a prototype changes).
   inline void ClearInstanceofCache();