Auto-compare GrProcessors' texture accesses in isEqual().

R=joshualitt@google.com

Review URL: https://codereview.chromium.org/654313002
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index 016780f..7e91af4 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -223,8 +223,7 @@
 
 bool AlphaThresholdEffect::onIsEqual(const GrFragmentProcessor& sBase) const {
     const AlphaThresholdEffect& s = sBase.cast<AlphaThresholdEffect>();
-    return (this->texture(0) == s.texture(0) &&
-            this->fInnerThreshold == s.fInnerThreshold &&
+    return (this->fInnerThreshold == s.fInnerThreshold &&
             this->fOuterThreshold == s.fOuterThreshold);
 }