Currently DebugInfo objects can be created independently from whether
the debugger is active. When tearing down the isolate, we would go
through DebugInfo objects and iterate through break locations,
causing this assertion to fail.

R=ulan@chromium.org
BUG=v8:4241
LOG=N

Review URL: https://codereview.chromium.org/1210813002

Cr-Commit-Position: refs/heads/master@{#29286}