Revert of replace kIgnore_SkAlphaType with kUnknown_SkAlphaType (patchset #3 id:40001...

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:

Review URL: https://codereview.chromium.org/966753002