Stepping in a generator now behaves similar to stepping inside an
async function. Stepping in or next at a yield expression will result in
a break inside the same generator when we return to the generator.
Behavior of step-out does not change.

R=jgruber@chromium.org, neis@chromium.org
BUG=chromium:496865

Review-Url: https://codereview.chromium.org/2519853002
Cr-Commit-Position: refs/heads/master@{#41132}