Add gpu implementation of OverdrawXfermode

src/gpu/effects/GrCustomXfermode.cpp

Index: src/gpu/effects/GrCustomXfermode.cpp
diff --git a/src/gpu/effects/GrCustomXfermode.cpp b/src/gpu/effects/GrCustomXfermode.cpp
index 04ff4a746ba733692ddab05ebb012f94ee341afb..459ff52948c361b76a7fbeb8d8886db7f058a4b7 100644
--- a/src/gpu/effects/GrCustomXfermode.cpp
+++ b/src/gpu/effects/GrCustomXfermode.cpp
@@ -172,17 +172,8 @@ private:
         GrGLSLBlend::AppendMode(fragBuilder, srcColor, dstColor, outColor, xp.mode());
 
         // Apply coverage.
-        if (xp.dstReadUsesMixedSamples()) {
-            if (srcCoverage) {
-                fragBuilder->codeAppendf("%s *= %s;", outColor, srcCoverage);
-                fragBuilder->codeAppendf("%s = %s;", outColorSecondary, srcCoverage);
-            } else {
-                fragBuilder->codeAppendf("%s = vec4(1.0);", outColorSecondary);
-            }
-        } else if (srcCoverage) {
-            fragBuilder->codeAppendf("%s = %s * %s + (vec4(1.0) - %s) * %s;",
-                                     outColor, srcCoverage, outColor, srcCoverage, dstColor);
-        }
+        INHERITED::DefaultCoverageModulation(fragBuilder, srcCoverage, dstColor, outColor,
+                                             outColorSecondary, xp);
     }
 
     void onSetData(const GrGLSLProgramDataManager&, const GrXferProcessor&) override {}