The StringIndexOf operation is pure on the JS level, but the actual stub
call must be in the effect chain later so that the Scheduler doesn't
place it inside some allocation region (The %StringIndexOf runtime
function may trigger a GC for string flattening).

BUG=chromium:685580
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2657243002
Cr-Commit-Position: refs/heads/master@{#42736}