Add support for unpremul image sources in the GPU backend

Change-Id: Ib940fa0d658fff0b2dd923abfcc5ca4a49b2b38b
Reviewed-on: https://skia-review.googlesource.com/148397
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/src/gpu/GrYUVProvider.cpp b/src/gpu/GrYUVProvider.cpp
index 3c6c7f7..9ffd8a7 100644
--- a/src/gpu/GrYUVProvider.cpp
+++ b/src/gpu/GrYUVProvider.cpp
@@ -130,7 +130,7 @@
     // If the caller expects the pixels in a different color space than the one from the image,
     // apply a color conversion to do this.
     std::unique_ptr<GrFragmentProcessor> colorConversionProcessor =
-            GrColorSpaceXformEffect::Make(srcColorSpace, dstColorSpace);
+            GrColorSpaceXformEffect::Make(srcColorSpace, kPremul_SkAlphaType, dstColorSpace);
     if (colorConversionProcessor) {
         paint.addColorFragmentProcessor(std::move(colorConversionProcessor));
     }