Remove SK_SUPPORT_LEGACY_GRADIENT_DITHERING from Skia proper

Migrating the flag to embedder defines (Chromium already guarded).

Also augment gradient-focused GMs to generate both dithered/undithered
results.

BUG=skia:4436
R=reed@google.com,robertphillips@google.com

Review URL: https://codereview.chromium.org/1400813006
6 files changed