Re-land rev. 12849 and 12868 (Heavy cleanup of the external pointer API + related fix).

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index ae14302306ed06ca9a205af214f8f621ca36e7c8..eecb4362d0820939439d873c9aef8f03d26ccef5 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -155,7 +155,8 @@ namespace internal {
   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(JSObject, observation_state, ObservationState)
+  V(JSObject, observation_state, ObservationState)                             \
+  V(Map, external_map, ExternalMap)
 
 #define ROOT_LIST(V)                                  \
   STRONG_ROOT_LIST(V)                                 \
@@ -665,6 +666,9 @@ 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();