Make skpaint->grpaint flow work for composing draws

include/gpu/effects/GrXfermodeFragmentProcessor.h

Index: include/gpu/effects/GrXfermodeFragmentProcessor.h
diff --git a/include/gpu/effects/GrXfermodeFragmentProcessor.h b/include/gpu/effects/GrXfermodeFragmentProcessor.h
index e2d1c0a544326c414ad861ca97d89b74a184f70d..fb07d003cd65426c79e4ba97de1665268280d849 100644
--- a/include/gpu/effects/GrXfermodeFragmentProcessor.h
+++ b/include/gpu/effects/GrXfermodeFragmentProcessor.h
@@ -23,6 +23,9 @@ namespace GrXfermodeFragmentProcessor {
     const GrFragmentProcessor* CreateFromSrcProcessor(const GrFragmentProcessor* src,
                                                       SkXfermode::Mode mode);
 
+    /** Takes the input color, which is assumed to be unpremultiplied, passes it as an opaque color
+        to both src and dst. The outputs of a src and dst are blended using mode and the original
+        input's alpha is applied to the blended color to produce a premul output. */
     const GrFragmentProcessor* CreateFromTwoProcessors(const GrFragmentProcessor* src,
                                                        const GrFragmentProcessor* dst,
                                                        SkXfermode::Mode mode);