Plumb through need for unpremul/premul

Update unittest to build test image with legal premul pixels.

Bug: skia:
Change-Id: Iebd1d2f81cac77f8913bd79f6ac25983ed710641
Reviewed-on: https://skia-review.googlesource.com/21735
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/src/gpu/GrYUVProvider.cpp b/src/gpu/GrYUVProvider.cpp
index 5ab926b..df71139 100644
--- a/src/gpu/GrYUVProvider.cpp
+++ b/src/gpu/GrYUVProvider.cpp
@@ -148,7 +148,8 @@
         if (ctx->caps()->srgbWriteControl()) {
             paint.setDisableOutputConversionToSRGB(true);
         } else {
-            paint.addColorFragmentProcessor(GrSRGBEffect::Make(GrSRGBEffect::Mode::kSRGBToLinear));
+            paint.addColorFragmentProcessor(GrSRGBEffect::Make(GrSRGBEffect::Mode::kSRGBToLinear,
+                                                               GrSRGBEffect::Alpha::kOpaque));
         }
     }