Reduce size of second iteration in SkTileGridNextDatum<>().
Reduce size of second iteration in SkTileGridNextDatum<>().

Rather than iterating over the entire dataset twice, during the first pass
track how large the second pass needs to be. Entirely data-dependent but
in practice approaches 2x speedup.

BUG=1212
R=junov
https://codereview.appspot.com/8315044/



csulmone/skia,csulmone/skia,csulmone/skia,csulmone/skia