Use child processors to implement compose color filter.

include/core/SkColorFilter.h

Index: include/core/SkColorFilter.h
diff --git a/include/core/SkColorFilter.h b/include/core/SkColorFilter.h
index a92f9aeff2899e2807e3966798868974a018dd99..a5a1a4c0cc546d8229e4c3481a0212cbd52af696 100644
--- a/include/core/SkColorFilter.h
+++ b/include/core/SkColorFilter.h
@@ -124,20 +124,17 @@ public:
     static SkColorFilter* CreateComposeFilter(SkColorFilter* outer, SkColorFilter* inner);
 
     /**
-     *  A subclass may implement this factory function to work with the GPU backend.
-     *  If it returns true, then 1 or more fragment processors will have been appended to the
-     *  array, each of which has been ref'd, so that the caller is responsible for calling unref()
-     *  on them when they are finished. If more than one processor is appended, they will be
-     *  applied in FIFO order.
+     *  A subclass may implement this factory function to work with the GPU backend. It returns
+     *  a GrFragmentProcessor that implemets the color filter in GPU shader code.
      *
-     *  The fragment processor(s) must each return their color as a premul normalized value
-     *  e.g. each component between [0..1] and each color component <= alpha.
+     *  The fragment processor receives a premultiplied input color and produces a premultiplied
+     *  output color.
      *
-     *  If the subclass returns false, then it should not modify the array at all.
+     *  A null return indicates that the color filter isn't implemented for the GPU backend.
      */
-    virtual bool asFragmentProcessors(GrContext*, GrProcessorDataManager*,
-                                      SkTDArray<const GrFragmentProcessor*>*) const {
-        return false;
+    virtual const GrFragmentProcessor* asFragmentProcessor(GrContext*,
+                                                           GrProcessorDataManager*) const {
+        return nullptr;
     }
 
     bool affectsTransparentBlack() const {