Migrate newly added C++ String Iterator builtins to TFJ builtins, per
step 4. of the String Iterator Baseline Implementation section of the design doc

BUG=v8:5388
R=bmeurer@chromium.org, mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2358263002
Cr-Commit-Position: refs/heads/master@{#39765}