Debugger: make debug code on-stack replacement more robust.

Debugger: make debug code on-stack replacement more robust.

The new implemtation counts the number of calls (or continuations)
before the PC to find the corresponding PC in the new code.

R=mstarzinger@chromium.org
BUG=chromium:507070
LOG=N

Review URL: https://codereview.chromium.org/1235603002

Cr-Commit-Position: refs/heads/master@{#29636}