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, jkummerow@chromium.org

Committed: https://crrev.com/f9a2c8b1112c4e915df8bc5f7ea1fccdf7a33ff8
Cr-Commit-Position: refs/heads/master@{#39765}

patch from issue 2358263002 at patchset 260001 (http://crrev.com/2358263002#ps260001)

Review-Url: https://codereview.chromium.org/2381053002
Cr-Commit-Position: refs/heads/master@{#39879}