Add gpu backend for SkPixelXorXfermode

src/gpu/glsl/GrGLSLXferProcessor.cpp

Index: src/gpu/glsl/GrGLSLXferProcessor.cpp
diff --git a/src/gpu/glsl/GrGLSLXferProcessor.cpp b/src/gpu/glsl/GrGLSLXferProcessor.cpp
index 81bd015718c8e8ac7aeea99b76998409d8c252b2..e109775aced5e2bf16db58e998f85c20a8351895 100644
--- a/src/gpu/glsl/GrGLSLXferProcessor.cpp
+++ b/src/gpu/glsl/GrGLSLXferProcessor.cpp
@@ -88,3 +88,21 @@ void GrGLSLXferProcessor::setData(const GrGLSLProgramDataManager& pdm, const GrX
     this->onSetData(pdm, xp);
 }
 
+void GrGLSLXferProcessor::DefaultCoverageModulation(GrGLSLXPFragmentBuilder* fragBuilder,
+                                                    const char* srcCoverage,
+                                                    const char* dstColor,
+                                                    const char* outColor,
+                                                    const char* outColorSecondary,
+                                                    const GrXferProcessor& proc) {
+    if (proc.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);
+    }
+}