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..956ba6e00d2ad003ae27817160bb359003d3d5c0 100644
--- a/include/core/SkComposeShader.h
+++ b/include/core/SkComposeShader.h
@@ -36,6 +36,10 @@ public:
 
     size_t contextSize() const override;
 
+    bool asFragmentProcessor(GrContext*, const SkPaint&, const SkMatrix& viewM, const SkMatrix*,
+                             GrColor*, GrProcessorDataManager*,
+                             GrFragmentProcessor**) const override;
+
     class ComposeShaderContext : public SkShader::Context {
     public:
         // When this object gets destroyed, it will call contextA and contextB's destructor