support external raster handles

draft CL for chrome: https://codereview.chromium.org/2618323005/

BUG=skia:

Change-Id: I5dbcd700818776a9f62f1e10723d2efcc248dc44
Reviewed-on: https://skia-review.googlesource.com/6406
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/include/utils/mac/SkCGUtils.h b/include/utils/mac/SkCGUtils.h
index 592f2f6..2dcbb96 100644
--- a/include/utils/mac/SkCGUtils.h
+++ b/include/utils/mac/SkCGUtils.h
@@ -24,8 +24,11 @@
 
 class SkBitmap;
 class SkData;
+class SkPixmap;
 class SkStreamRewindable;
 
+SK_API CGContextRef SkCreateCGContext(const SkPixmap&);
+
 /**
  *  Given a CGImage, allocate an SkBitmap and copy the image's pixels into it. If scaleToFit is not
  *  null, use it to determine the size of the bitmap, and scale the image to fill the bitmap.