Ensure SkPictureImageFilter::onFilterImage doesn't double-convert color

Consider the following sequence of events:
1. SkPictureImageFilter::onFilterImage creates a local canvas, wraps it
   in a SkColorSpaceXformCanvas, and passed to...
2. SkPictureImageFilter::drawPictureAtLocalResolution creates a local
   canvas (localCanvas) wraps that in a SkColorSpaceXformCanvas, draws
   the picture to it, doing sRGB->fColorSpace conversion
3. We then call makeImageSnapshot to produce a SkSpecialImage, localImg,
   which is not tagged with any color space.
4. When the draw that localImg to the passed-in SkColorSpaceXformCanvas,
   which then performs sRGB->fColorSpace conversion a second time.

We now have performed color conversion twice.

One fix for this would be to have the image produced by the call to
localSurface->makeImageSnapshot() be tagged with fColorSpace. This is
somewhat involved.

The less invasive fix is to remove the SkColorSpaceXformCanvas in
SkPictureImageFilter::onFilterImage, and push it down into the two
branches, SkPictureImageFilter::drawPictureAtLocalResolution and
SkPictureImageFilter::drawPictureAtDeviceResolution.

BUG=728332

Change-Id: If2aa32e18ad660b3e361f1d90845eeb8555fe404
Reviewed-on: https://skia-review.googlesource.com/18282
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Christopher Cameron <ccameron@google.com>
Commit-Queue: Christopher Cameron <ccameron@google.com>

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