SkPDF: SkPDFString is no longer aware of wide strings.

Since wide strings are only used in SkPDFDevice, I have moved the
function that manages them to that directory.

Motivation: The SkPDFString will be refactored later along with the
other SkPFObject heirarchy.

BUG=skia:3585

Review URL: https://codereview.chromium.org/1064013003
3 files changed