Use a blink-specific slot in the per-isolate data

BUG=321631
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/82823003

git-svn-id: svn://svn.chromium.org/blink/trunk@162533 bbb929c8-8fbe-4397-9dbb-9b2b20218538