Start canonicalizing color for all A8 textblobs

include/core/SkPaint.h

Index: include/core/SkPaint.h
diff --git a/include/core/SkPaint.h b/include/core/SkPaint.h
index 991a727f935498cbf9d716313e1089d034026221..e4be06e03e8207b802a509e79ee37c7f88fa11f5 100644
--- a/include/core/SkPaint.h
+++ b/include/core/SkPaint.h
@@ -1076,6 +1076,13 @@ private:
                         void (*proc)(SkTypeface*, const SkDescriptor*, void*),
                         void* context, bool ignoreGamma = false) const;
 
+    /*
+     * The luminance color is used to determine which Gamma Canonical color to map to.  This is
+     * really only used by backends which want to cache glyph masks, and need some way to know if
+     * they need to generate new masks based off a given color.
+     */
+    SkColor computeLuminanceColor() const;
+
     static void Term();
 
     enum {
@@ -1129,6 +1136,7 @@ private:
     friend class GrPathRendering;
     friend class GrTextContext;
     friend class GrGLPathRendering;
+    friend class SkScalerContext;
     friend class SkTextToPathIter;
     friend class SkCanonicalizePaint;
 };