Break GrTextContext's reliance on SkGpuDevice

This CL seems to have 2 main downsides:

1) It duplicates some code in SkBaseDevice::filterTextFlags
2) It makes it tougher to derive from SkGpuDevice

It seems reasonable (at least to me) that the TextContexts get the power to reset the LCD flags.

Review URL: https://codereview.chromium.org/1159973002
6 files changed