Debugger: use debug break slots to break at function exit.

Debugger: use debug break slots to break at function exit.

By not having to patch the return sequence (we patch the debug
break slot right before it), we don't overwrite it and therefore
don't have to keep the original copy of the code around.

R=ulan@chromium.org
BUG=v8:4269
LOG=N

Review URL: https://codereview.chromium.org/1234833003

Cr-Commit-Position: refs/heads/master@{#29672}