Make XPFragmentBuilder only Builder with access to DstCopy.

src/effects/SkColorCubeFilter.cpp

Index: src/effects/SkColorCubeFilter.cpp
diff --git a/src/effects/SkColorCubeFilter.cpp b/src/effects/SkColorCubeFilter.cpp
index 49ce8401cb72dd88383e2b401e51fe57a2e4c159..3878e5fc197869dae8b1ec4accdbc7cd8283465d 100644
--- a/src/effects/SkColorCubeFilter.cpp
+++ b/src/effects/SkColorCubeFilter.cpp
@@ -295,7 +295,7 @@ void GrColorCubeEffect::GLProcessor::emitCode(GrGLFPBuilder* builder,
     // Note: if implemented using texture3D in OpenGL ES older than OpenGL ES 3.0,
     //       the shader might need "#extension GL_OES_texture_3D : enable".
 
-    GrGLFPFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
+    GrGLFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
 
     // Unpremultiply color
     fsBuilder->codeAppendf("\tfloat %s = max(%s.a, 0.00001);\n", nonZeroAlpha, inputColor);