Use child processors to implement compose color filter.

include/gpu/GrFragmentProcessor.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrFragmentProcessor_DEFINED
 #define GrFragmentProcessor_DEFINED
 
@@ skipping 23 matching lines @@
     static const GrFragmentProcessor* MulOutputByInputAlpha(const GrFragmentProcessor*);
 
     /**
      *  Similar to the above but it modulates the output r,g,b of the child processor by the input
      *  rgb and then multiplies all the components by the input alpha. This effectively modulates
      *  the child processor's premul color by a unpremul'ed input and produces a premul output
      */
     static const GrFragmentProcessor* MulOutputByInputUnpremulColor(const GrFragmentProcessor*);
 
     /**
-     * Returns a parent fragment processor that adopts the passed fragment processor as a child. The
-     * parent will ignore its input color and instead feed the passed in color as input to the
-     * child.
+     *  Returns a parent fragment processor that adopts the passed fragment processor as a child.
+     *  The parent will ignore its input color and instead feed the passed in color as input to the
+     *  child.
      */
     static const GrFragmentProcessor* OverrideInput(const GrFragmentProcessor*, GrColor);
 
+    /**
+     * Returns a fragment processor that runs the passed in array of fragment processors in a
+     * series. The original input is passed to the first, the first's output is passed to the
+     * second, etc. The output of the returned processor is the output of the last processor of the
+     * series.
+     */
+    static const GrFragmentProcessor* RunInSeries(const GrFragmentProcessor*[], int cnt);
+
     GrFragmentProcessor()
         : INHERITED()
         , fUsesLocalCoords(false)
         , fNumTexturesExclChildren(0)
         , fNumTransformsExclChildren(0) {}
 
     ~GrFragmentProcessor() override;
 
     GrGLFragmentProcessor* createGLInstance() const;
 
@@ skipping 145 matching lines @@
     int                                          fNumTransformsExclChildren;
 
     // TODO: These must convert their processors to pending-execution refs when the parent is
     // converted (do this automatically in GrProgramElement?).
     SkTArray<const GrFragmentProcessor*, true>   fChildProcessors;
 
     typedef GrProcessor INHERITED;
 };
 
 #endif