Push isEqual/onIsEqual down from GrProcessor to subclasses.

R=joshualitt@google.com

Review URL: https://codereview.chromium.org/654273002
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index af21ffc..b297b14 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -1180,7 +1180,7 @@
     }
 }
 
-bool GrGradientEffect::onIsEqual(const GrProcessor& processor) const {
+bool GrGradientEffect::onIsEqual(const GrFragmentProcessor& processor) const {
     const GrGradientEffect& s = processor.cast<GrGradientEffect>();
 
     if (this->fColorType == s.getColorType()){