Revert "remove unused api on xfermodeimagefilter"

This reverts commit fcc4a071d9dff4f3bac0bd55dab8f69a4436d15d.

Reason for revert: broke google3 roll.

Original change's description:
> remove unused api on xfermodeimagefilter
> 
> Bug: skia:
> Change-Id: If99ee7b4d959d728849a20ee43a0d0ec25196f58
> Reviewed-on: https://skia-review.googlesource.com/20303
> Reviewed-by: Mike Reed <reed@google.com>
> Commit-Queue: Mike Reed <reed@google.com>

TBR=reed@google.com
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:

Change-Id: I80e8b1e9009263a61230d22a988a9bb5ba7e162d
Reviewed-on: https://skia-review.googlesource.com/20320
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
1 file changed