This invariant will save us some head ache.
The changes to test-debug/DebugStub is due to the fact that it abuses
the ability to set break points in code that has no debug break slots.
This is now no longer possible.

R=ulan@chromium.org
BUG=v8:4132
LOG=N

Review URL: https://codereview.chromium.org/1181013007

Cr-Commit-Position: refs/heads/master@{#29038}