Push isEqual/onIsEqual down from GrProcessor to subclasses.

R=joshualitt@google.com

Review URL: https://codereview.chromium.org/654273002
diff --git a/src/gpu/effects/GrDitherEffect.cpp b/src/gpu/effects/GrDitherEffect.cpp
index 96cdd75..0e7df5e 100644
--- a/src/gpu/effects/GrDitherEffect.cpp
+++ b/src/gpu/effects/GrDitherEffect.cpp
@@ -40,7 +40,7 @@
     }
 
     // All dither effects are equal
-    virtual bool onIsEqual(const GrProcessor&) const SK_OVERRIDE { return true; }
+    virtual bool onIsEqual(const GrFragmentProcessor&) const SK_OVERRIDE { return true; }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE;