Some distance field text optimizations.

- Add clipRect check
- Remove creation of scalerContext to check for color fonts in canDraw()
   (no longer needed)

BUG=skia:2933

Review URL: https://codereview.chromium.org/696503004
Beginning to refactor nvpr code

BUG=skia:

Review URL: https://codereview.chromium.org/687563008
GrContext no longer ever draws on Gpu

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/7a75e5230872732e71ea99058d9afe5d4b6758b0

Review URL: https://codereview.chromium.org/683733005