Standardize naming on CoverageAsAlpha (rather than AlphaAsCoverage)

Also fix a spelling mistake: Coveage -> Coverage

Bug: skia:
Change-Id: I296415ca194b0805470596d297e1d815f43b404a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201394
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
13 files changed