Replaced all calls to fragmentPosition() with sk_FragCoord

BUG=skia:

Change-Id: I179576e148ea6caf6e1c40f0a216421898bcb35d
Reviewed-on: https://skia-review.googlesource.com/5941
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
32 files changed