Reland "implement SkPixmap::erase() with draws"

This is a reland of 42f29f11817f9829b92dbf52d77f1447fbfc5ed0

The original had to be reverted because of a buggy Chromium unit test,
now fixed.  I also took the opportunity to implement this with SkDraw
rather than SkCanvas.

Original change's description:
> implement SkPixmap::erase() with draws
>
> I don't want to have to maintain yet another switch on SkColorType.
>
> This should also make color management a bit clearer?
>
> Change-Id: I1508e0c30670b1987972f7eeba2dd6d23591c531
> Reviewed-on: https://skia-review.googlesource.com/154862
> Commit-Queue: Mike Klein <mtklein@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>

Change-Id: I6bc071e8a6025723d47f2ad5f6fb7f8fa5dccc2b
Reviewed-on: https://skia-review.googlesource.com/155240
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed