Alter SkXfermode's asFragmentProcessor & asXPFactory contracts

TBR=bsalomon@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1674673002

Review URL: https://codereview.chromium.org/1674673002
diff --git a/src/utils/debugger/SkOverdrawMode.cpp b/src/utils/debugger/SkOverdrawMode.cpp
index 400e583..3b695ec 100644
--- a/src/utils/debugger/SkOverdrawMode.cpp
+++ b/src/utils/debugger/SkOverdrawMode.cpp
@@ -291,19 +291,13 @@
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkOverdrawXfermode)
 
 #if SK_SUPPORT_GPU
-    bool asFragmentProcessor(const GrFragmentProcessor** output,
-                             const GrFragmentProcessor* dst) const override {
-        if (output) {
-            *output = GrOverdrawFP::Create(dst);
-        }
-        return true;
+    const GrFragmentProcessor* getFragmentProcessorForImageFilter(
+                                                const GrFragmentProcessor* dst) const override {
+        return GrOverdrawFP::Create(dst);
     }
 
-    bool asXPFactory(GrXPFactory** xpf) const override {
-        if (xpf) {
-            *xpf = GrOverdrawXPFactory::Create();
-        }
-        return true;
+    GrXPFactory* asXPFactory() const override {
+        return GrOverdrawXPFactory::Create();
     }
 #endif