Heavy cleanup of the external pointer API.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index da78b18a512764dae3510e6fcc6dfaa66665e129..297b24f7a60c97395210d67e1136c70234399ad2 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -154,7 +154,8 @@ namespace internal {
   V(Smi, arguments_adaptor_deopt_pc_offset, ArgumentsAdaptorDeoptPCOffset)     \
   V(Smi, construct_stub_deopt_pc_offset, ConstructStubDeoptPCOffset)           \
   V(Smi, getter_stub_deopt_pc_offset, GetterStubDeoptPCOffset)                 \
-  V(Smi, setter_stub_deopt_pc_offset, SetterStubDeoptPCOffset)
+  V(Smi, setter_stub_deopt_pc_offset, SetterStubDeoptPCOffset)                 \
+  V(Map, external_map, ExternalMap)
 
 #define ROOT_LIST(V)                                  \
   STRONG_ROOT_LIST(V)                                 \
@@ -664,6 +665,9 @@ class Heap {
   // Allocates a serialized scope info.
   MUST_USE_RESULT MaybeObject* AllocateScopeInfo(int length);
 
+  // Allocates an External object for v8's external API.
+  MUST_USE_RESULT MaybeObject* AllocateExternal(void* value);
+
   // Allocates an empty PolymorphicCodeCache.
   MUST_USE_RESULT MaybeObject* AllocatePolymorphicCodeCache();