Move setAlphaAndRequiredFrame to src/codec

The corresponding declaration is already in src/codec/SkFrameHolder.h

That code is not solely used by third_party/gif, and in the future, we
may have another GIF implementation.

Bug: skia:
Change-Id: If04cfd4a5b1106f380c89e11c34a8d854ce2b05a
Reviewed-on: https://skia-review.googlesource.com/144240
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
2 files changed