Make XPFragmentBuilder only Builder with access to DstCopy.

src/effects/SkColorMatrixFilter.cpp

Index: src/effects/SkColorMatrixFilter.cpp
diff --git a/src/effects/SkColorMatrixFilter.cpp b/src/effects/SkColorMatrixFilter.cpp
index 1c9eef9978d1255e130c2385b0df224dc8e372f3..58f074c9aa19f2da9a80e4707e6a7f5e49a54865 100644
--- a/src/effects/SkColorMatrixFilter.cpp
+++ b/src/effects/SkColorMatrixFilter.cpp
@@ -444,7 +444,7 @@ public:
                 // could optimize this case, but we aren't for now.
                 inputColor = "vec4(1)";
             }
-            GrGLFPFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
+            GrGLFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
             // The max() is to guard against 0 / 0 during unpremul when the incoming color is
             // transparent black.
             fsBuilder->codeAppendf("\tfloat nonZeroAlpha = max(%s.a, 0.00001);\n", inputColor);