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

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index c86c0b2f431bd08890b7250d4d59d30e770f7e25..3dc6188997bdead58671bc9982202033e64f5e38 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -907,6 +907,7 @@ class Object : public MaybeObject {
 #undef IS_TYPE_FUNCTION_DECL
 
   inline bool IsFixedArrayBase();
+  inline bool IsExternal();
 
   // Returns true if this object is an instance of the specified
   // function template.
@@ -2451,6 +2452,8 @@ class FixedArray: public FixedArrayBase {
                                                   Object* value);
 
  private:
+  STATIC_CHECK(kHeaderSize == Internals::kFixedArrayHeaderSize);
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(FixedArray);
 };