Use kIgnore blend behavior when encoding JPEG

This looks to have been an oversight - encoding PNG and WEBP both use
kIgnore when called through this interface.

Bug: b/65262488
Change-Id: I23ba02f979210087808ee2da026a2ca0269e0fb4
Reviewed-on: https://skia-review.googlesource.com/43261
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
1 file changed