Add ability to extract YUV planes from SkImage

src/core/SkColorMatrixFilterRowMajor255.h

Index: src/core/SkColorMatrixFilterRowMajor255.h
diff --git a/src/core/SkColorMatrixFilterRowMajor255.h b/src/core/SkColorMatrixFilterRowMajor255.h
index 453dd41bff5d168250fb45fb9e2568db53a368e4..106d2bd882a798886824c050526a1ba70f7f0e48 100644
--- a/src/core/SkColorMatrixFilterRowMajor255.h
+++ b/src/core/SkColorMatrixFilterRowMajor255.h
@@ -14,6 +14,9 @@ class SK_API SkColorMatrixFilterRowMajor255 : public SkColorFilter {
 public:
     explicit SkColorMatrixFilterRowMajor255(const SkScalar array[20]);
 
+    /** Creates a color matrix filter that returns the same value in all four channels. */
+    static SkColorFilter* CreateSingleChannelOutput(const SkScalar row[5]);
+
     void filterSpan(const SkPMColor src[], int count, SkPMColor[]) const override;
     void filterSpan4f(const SkPM4f src[], int count, SkPM4f[]) const override;
     uint32_t getFlags() const override;
@@ -32,11 +35,13 @@ protected:
     void flatten(SkWriteBuffer&) const override;
 
 private:
+    SkColorMatrixFilterRowMajor255() {};
+
     SkScalar        fMatrix[20];
     float           fTranspose[20]; // for Sk4s
     uint32_t        fFlags;
 
-    void initState(const SkScalar array[20]);
+    void initState();
 
     typedef SkColorFilter INHERITED;
 };