port fc9c5275 (r29672).

original commit message:

    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.

BUG=

Review URL: https://codereview.chromium.org/1236023007

Cr-Commit-Position: refs/heads/master@{#29694}