We used to flood the handler when preparing for stepping,
even if we may not throw. Instead, we now flood the
handler only when we actually throw.

This also solves an issue with step-next when we throw and
leave the function unexpectedly. In combination with
microtasks, this could cause a crash.

R=mstarzinger@chromium.org
BUG=chromium:568477
LOG=N

Review URL: https://codereview.chromium.org/1527593002

Cr-Commit-Position: refs/heads/master@{#32856}