wasm_to_js and js_to_wasm both derive from wasm, which was confusing because is_wasm wasn't true for them and that made WasmFrame::cast awkward. Make them derive from StubFrame instead.

R=bradnelson@chromium.org, titzer@chromium.org

Review URL: https://codereview.chromium.org/1839843002

Cr-Commit-Position: refs/heads/master@{#35115}