Port fc9c5275

Original commit message:
    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=yangguo@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1238503003

Cr-Commit-Position: refs/heads/master@{#29703}