This also fixes bugs found by the new test. It only tests stepping
inside of wasm code. Wasm to JS and vice versa will follow in another
CL.

R=yangguo@chromium.org, titzer@chromium.org, kozyatinskiy@chromium.org
BUG=v8:5822

Review-Url: https://codereview.chromium.org/2651043004
Cr-Commit-Position: refs/heads/master@{#42729}