Revert of Revert of replace kIgnore_SkAlphaType with kUnknown_SkAlphaType (patchset...

Revert of Revert of replace kIgnore_SkAlphaType with kUnknown_SkAlphaType (patchset #1 id:1 of https://codereview.chromium.org/966753002/)

Reason for revert:
Android has been updated, so we can re-land this.

Original issue's description:
> Revert of replace kIgnore_SkAlphaType with kUnknown_SkAlphaType (patchset #3 id:40001 of https://codereview.chromium.org/964613002/)
>
> Reason for revert:
> This breaks Android framework build. See
>
> https://android-build.storage.googleapis.com/builds/git_master-skia-linux-volantisg-userdebug/1759130/7f0fcdbf3c0d47530d770a15c912298dac20dd7d90a01de63ecb9ba16c96b25c/logs/build_error.log?Expires=1425047630&GoogleAccessId=701025073339-mqn0q2nvir9iurm6q5d00tdv7blbgvjr%40developer.gserviceaccount.com&Signature=BHPS1lffyH7WykwyEw5RXVVXWqQK5rab%2B6Pl1VaUrTlmWT%2FXb6fq%2FBe22AUDZYFPG24MBR%2F2IzIDzTqH3mihQS9wIkcKJ4kJpA2G%2BI3l8jCX20brZSr3bsDSx%2F4wsLwU1bBpIxI%2FbrauoPRqddbXP8sNI1l51u6ZCxpvRQms9Mc%3D
>
> We need to stop them from using kIgnore before we can reland this.
>
> Original issue's description:
> > replace kIgnore_SkAlphaType with kUnknown_SkAlphaType
> >
> > BUG=skia:
> > TBR=
> >
> > Committed: https://skia.googlesource.com/skia/+/1c8aeeaebce9675197be31bd769e8ffa2531bf86
>
> TBR=reed@google.com,reed@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/574290f61a47bd1ce1f9e2d941533bda9c8f03fe

TBR=reed@chromium.org,scroggo@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/966853002