Remove SK_LEGACY_FRAME_INFO_ALPHA_TYPE

With https://chromium-review.googlesource.com/c/621815, Chromium no
longer references this. Delete it.

TBR=reed@google.com
Bug: skia:6839
Change-Id: Icd3d394858859e6732b1429394c7c04bb22b9dea
Reviewed-on: https://skia-review.googlesource.com/36581
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
3 files changed