This bug was triggered by a very specific combination:

* A context-allocated variable at script scope.
* OSR optimization.
* A scheduled breakpoint, which triggers at stack checks.

Stack checks differ from other possible breakpoint locations in that
the context (among other things) may be in a register and not on the
stack, making it impossible to recover during deoptimization. The
frame_inspector then returns undefined when asked for the context.

In GetFrameDetails, handle this case by omitting all context-allocated
variables.

BUG=v8:5279

Review-Url: https://codereview.chromium.org/2245603002
Cr-Commit-Position: refs/heads/master@{#38611}