Make sure to flatten strings first in JSON.parse() builtins, otherwise
we always hit the slow path for non-sequential strings, i.e. for cons
strings.

Also don't create any arguments adaptor frames for JSON.parse() as the
C++ builtin can handle any number of inputs properly.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2039553002
Cr-Commit-Position: refs/heads/master@{#36722}