Heavy cleanup of the external pointer API.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 0d1a69cb9882985fa282c4e2f3dc4a5fa98c9523..50018bfb70e0f1ccc59858ae611a5b12f1f7b544 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -887,6 +887,7 @@ 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.
@@ -2410,6 +2411,8 @@ class FixedArray: public FixedArrayBase {
                                                   Object* value);
 
  private:
+  STATIC_CHECK(kHeaderSize == Internals::kFixedArrayHeaderSize);
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(FixedArray);
 };