harmony-scoping: Implement LoadIC handler for loads from global contexts.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index 8182532039996035798764d6f4fb56f0fd5f8a5c..a859a0bf25e351340a77dad804e87a2e5b74e63b 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -229,8 +229,14 @@ class GlobalContextTable : public FixedArray {
   static Handle<GlobalContextTable> Extend(Handle<GlobalContextTable> table,
                                            Handle<Context> global_context);
 
+  static int GetContextOffset(int context_index) {
+    return kFirstContextOffset + context_index * kPointerSize;
+  }
+
  private:
   static const int kUsedSlot = 0;
+  static const int kFirstContextOffset =
+      FixedArray::kHeaderSize + (kUsedSlot + 1) * kPointerSize;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(GlobalContextTable);
 };