Unpremul the blend constant in PDLCDXferProcessor.

This was accidentally dropped in https://skia.googlesource.com/skia.git/+/eec6f7be5461e588210f383b8af18f324a2bdb46

BUG=chromium:695626

Change-Id: If715fdc673fafa4baf6a61545c2b3b9d3ded9232
Reviewed-on: https://skia-review.googlesource.com/9117
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/effects/GrPorterDuffXferProcessor.cpp b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
index 4813c9d..7dd7be6 100644
--- a/src/gpu/effects/GrPorterDuffXferProcessor.cpp
+++ b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
@@ -633,6 +633,7 @@
     if (!analysis.hasKnownOutputColor(&blendConstant)) {
         return nullptr;
     }
+    blendConstant = GrUnpremulColor(blendConstant);
     uint8_t alpha = GrColorUnpackA(blendConstant);
     blendConstant |= (0xff << GrColor_SHIFT_A);
     return new PDLCDXferProcessor(blendConstant, alpha);