Decouple contrast boost from fake gamma.

Replace FakeGamma with FontWeightFlags. Largely mechanical, except for
the changes in ignorePreBlend. With this change, text GMs generally look
the same (or at least more similar) in 8888/srgb/gpu/gpusrgb configs.

No public API changes.
TBR=reed@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1866293003

Review URL: https://codereview.chromium.org/1866293003
15 files changed