Revert "make it illegal to include SkXfermode.h"

Revert "make it illegal to include SkXfermode.h"

This reverts commit 07764cefbb18041a77897df3453903b0a2016583.

Reason for revert: breaking google3

Original change's description:
> make it illegal to include SkXfermode.h
>
> BUG=skia:
>
> GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5133
>
> Change-Id: I6e8596dcb17cd7e8efa67859bb682bf9bfcac4db
> Reviewed-on: https://skia-review.googlesource.com/5133
> Reviewed-by: Mike Reed <reed@google.com>
> Commit-Queue: Mike Reed <reed@google.com>
>

TBR=reed@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I136f9e533eb60633c49dffa19b5747d50b6d98a8
Reviewed-on: https://skia-review.googlesource.com/5196
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>