SkWebpEncoder: use bgra for lossless and yuv for lossy

Previosuly, we would (accidentally) always use just yuv.

Bug: 713862
Change-Id: I00acc6ca2841ba0636494119b7b4f46a9deee401
Reviewed-on: https://skia-review.googlesource.com/17406
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
2 files changed