Added SkComposeShader GPU implementation

include/core/SkComposeShader.h

Index: include/core/SkComposeShader.h
diff --git a/include/core/SkComposeShader.h b/include/core/SkComposeShader.h
index dd10447a04880760a59b88d75eac4d7c0fd85480..41be197eb7769f766a77da2ddb866a9b2df43346 100644
--- a/include/core/SkComposeShader.h
+++ b/include/core/SkComposeShader.h
@@ -36,6 +36,14 @@ public:
 
     size_t contextSize() const override;
 
+#if SK_SUPPORT_GPU
+    const GrFragmentProcessor*  asFragmentProcessor(GrContext*,
+                                                    const SkMatrix& viewM,
+                                                    const SkMatrix* localMatrix,
+                                                    SkFilterQuality,
+                                                    GrProcessorDataManager*) const override;
+#endif
+
     class ComposeShaderContext : public SkShader::Context {
     public:
         // When this object gets destroyed, it will call contextA and contextB's destructor