This fixes a bug introduced by r28826 (Unify decoding of deoptimization
translations, https://codereview.chromium.org/1136223004), where we
started leaking arguments marker sentinel to the debugger, which would
then cause crashes. This change replaces the sentinel with the undefined
value in the debugger-inspectable frame.

BUG=chromium:514362
LOG=n
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1263333002

Cr-Commit-Position: refs/heads/master@{#29971}