Link all global contexts into a weak list.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index e284454c8eba5b13160560e892e3e98d1cd10cca..29c966450dd21ec04a02f9a83a5bcc41f58f0121 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1760,6 +1760,7 @@ class FixedArray: public HeapObject {
   // Setters with less debug checks for the GC to use.
   inline void set_unchecked(int index, Smi* value);
   inline void set_null_unchecked(int index);
+  inline void set_unchecked(int index, Object* value, WriteBarrierMode mode);
 
   // Gives access to raw memory which stores the array's data.
   inline Object** data_start();