Make JSFunction::BodyDescriptor the only single place that knows how to iterate JSFunction's body.

src/heap/objects-visiting.cc

Index: src/heap/objects-visiting.cc
diff --git a/src/heap/objects-visiting.cc b/src/heap/objects-visiting.cc
index 6ca91445eb63166391db17c975f239b6d6795eb7..3d6cb7309553ce5d1336a41ab83d2e27f88875c2 100644
--- a/src/heap/objects-visiting.cc
+++ b/src/heap/objects-visiting.cc
@@ -238,8 +238,7 @@ void HeapObject::IterateBody(InstanceType type, int object_size,
       JSArrayBuffer::JSArrayBufferIterateBody(this, v);
       break;
     case JS_FUNCTION_TYPE:
-      reinterpret_cast<JSFunction*>(this)
-          ->JSFunctionIterateBody(object_size, v);
+      JSFunction::BodyDescriptor::IterateBody(this, object_size, v);
       break;
     case ODDBALL_TYPE:
       Oddball::BodyDescriptor::IterateBody(this, v);