Add in shader blending for CoverageSetOp XP

BUG=skia:

Review URL: https://codereview.chromium.org/1488213002
diff --git a/src/gpu/glsl/GrGLSLBlend.h b/src/gpu/glsl/GrGLSLBlend.h
index 25ae37b..c8047f8 100644
--- a/src/gpu/glsl/GrGLSLBlend.h
+++ b/src/gpu/glsl/GrGLSLBlend.h
@@ -8,6 +8,7 @@
 #ifndef GrGLBlend_DEFINED
 #define GrGLBlend_DEFINED
 
+#include "SkRegion.h"
 #include "SkXfermode.h"
 
 class GrGLSLFragmentBuilder;
@@ -19,6 +20,9 @@
      */
     void AppendMode(GrGLSLFragmentBuilder* fsBuilder, const char* srcColor,
                     const char* dstColor, const char* outColor, SkXfermode::Mode mode);
+
+    void AppendRegionOp(GrGLSLFragmentBuilder* fsBuilder, const char* srcColor,
+                        const char* dstColor, const char* outColor, SkRegion::Op regionOp);
 };
 
 #endif