Transform check for unpaired surrogates check in a function

[Vitor Roriz]

At ComplexTextController, we have logic that checks for unpaired surrogated pair. We should encapsulate that check in a function that explicitly says what we are checking for:

```
   U16_NEXT(buffer, i, bufferLength, baseCharacter);
    if (U_IS_SURROGATE(baseCharacter)) {
        markCount = 0;
        currentIndex += i;
        return false;
    }
```

[Radar WebKit Bug Importer]

<rdar://problem/125515961>