This is in preparation for wasm on the native heap. All the
aforementioned API needs is the address where the JIT-ed code starts.
This refactoring reduces the dependency of the API to just that.

Bug: v8:6876
Change-Id: I00bbb171398f581db41b8a74ab719e8ea4db52c4
Reviewed-on: https://chromium-review.googlesource.com/755624Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49204}