This moves deserializer setup boilerplate inside the deserializers, and
makes improper usage less likely. For instance:

  ObjectDeserializer deserializer(&scd);
  /* ... deserializer setup ... */
  MaybeHandle<HeapObject> obj = deserializer.Deserialize(isolate);
  /* ... result checks and casts ... */

has now become:

  /* All setup and casts inside deserializer, impossible to illegally
     use the same instance multiple times. */
  MaybeHandle<SharedFunctionInfo> maybe_result =
      ObjectDeserializer::DeserializeSharedFunctionInfo(
          isolate, &scd, source);

Bug: v8:6624
Change-Id: Id5a1848e024e89cf86e5292389ba7c89f31d8e6b
Reviewed-on: https://chromium-review.googlesource.com/604791
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47219}