This is a quick fix for the hole leaking from generators via the debugger's frame
inspection feature: when collecting the arguments, convert each hole to undefined.
In the long term, we probably want to remember and restore the actual arguments
rather than pushing these dummy arguments on each resume.

BUG=v8:5164

Review-Url: https://codereview.chromium.org/2122923003
Cr-Commit-Position: refs/heads/master@{#37544}