change colorfilter to return an array of frag processors

BUG=skia:

Review URL: https://codereview.chromium.org/973593002
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index 401a50e..ff6f7fa 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -662,9 +662,12 @@
             SkColor filtered = colorFilter->filterColor(skPaint.getColor());
             grPaint->setColor(SkColor2GrColor(filtered));
         } else {
-            SkAutoTUnref<GrFragmentProcessor> fp(colorFilter->asFragmentProcessor(context));
-            if (fp.get()) {
-                grPaint->addColorProcessor(fp);
+            SkTDArray<GrFragmentProcessor*> array;
+            if (colorFilter->asFragmentProcessors(context, &array)) {
+                for (int i = 0; i < array.count(); ++i) {
+                    grPaint->addColorProcessor(array[i]);
+                    array[i]->unref();
+                }
             }
         }
     }