Revert "Revert "Use float colors for blend constant""

This reverts commit 5a4f2a341a4a32d8f175afe1828a7796155dbcb8.

Bug: skia:
Change-Id: I706009343dc1976311a88d57800a140efd776861
Reviewed-on: https://skia-review.googlesource.com/c/168363
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/effects/GrCoverageSetOpXP.cpp b/src/gpu/effects/GrCoverageSetOpXP.cpp
index e7ac874..8e1dbbb 100644
--- a/src/gpu/effects/GrCoverageSetOpXP.cpp
+++ b/src/gpu/effects/GrCoverageSetOpXP.cpp
@@ -117,7 +117,7 @@
             blendInfo->fDstBlend = kZero_GrBlendCoeff;
             break;
     }
-    blendInfo->fBlendConstant = 0;
+    blendInfo->fBlendConstant = SK_PMColor4fTRANSPARENT;
 }
 
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