Revert of 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 459ff52948c361b76a7fbeb8d8886db7f058a4b7..04ff4a746ba733692ddab05ebb012f94ee341afb 100644
--- a/src/gpu/effects/GrCustomXfermode.cpp
+++ b/src/gpu/effects/GrCustomXfermode.cpp
@@ -172,8 +172,17 @@
         GrGLSLBlend::AppendMode(fragBuilder, srcColor, dstColor, outColor, xp.mode());
 
         // Apply coverage.
-        INHERITED::DefaultCoverageModulation(fragBuilder, srcCoverage, dstColor, outColor,
-                                             outColorSecondary, xp);
+        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);
+        }
     }
 
     void onSetData(const GrGLSLProgramDataManager&, const GrXferProcessor&) override {}