Introduce global contexts to represent lexical global scope(s).

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index ae9791469415d756d48e334684814eeb061957c9..4b8a23b17b36da59410f343fa5aa2b3fe4b5a946 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -130,6 +130,7 @@ namespace internal {
   V(Map, with_context_map, WithContextMap)                                     \
   V(Map, block_context_map, BlockContextMap)                                   \
   V(Map, module_context_map, ModuleContextMap)                                 \
+  V(Map, global_context_map, GlobalContextMap)                                 \
   V(Map, oddball_map, OddballMap)                                              \
   V(Map, message_object_map, JSMessageObjectMap)                               \
   V(Map, foreign_map, ForeignMap)                                              \
@@ -826,6 +827,10 @@ class Heap {
   // Allocate a native (but otherwise uninitialized) context.
   MUST_USE_RESULT MaybeObject* AllocateNativeContext();
 
+  // Allocate a global context.
+  MUST_USE_RESULT MaybeObject* AllocateGlobalContext(JSFunction* function,
+                                                     ScopeInfo* scope_info);
+
   // Allocate a module context.
   MUST_USE_RESULT MaybeObject* AllocateModuleContext(ScopeInfo* scope_info);