StringFromCodePointOperator needed to derive from Operator1<UnicodeEncoding>
rather than plain old Operator. This was thankfully caught by the CFI build.

R=bmeurer@chromium.org, caitp@chromium.org
BUG=v8:5498

Review-Url: https://codereview.chromium.org/2398403003
Cr-Commit-Position: refs/heads/master@{#40131}