Previously, the stack property was set up in JS as read-only; but since
it had a JS setter, writability was ignored and writing to stack was
possible.

This is no longer the case now that stack is either an actual data
property, or is associated with C++ accessors. Explicitly set the
property as writable to preserve old behavior.

BUG=5245
R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2190313002
Cr-Commit-Position: refs/heads/master@{#38158}