Add toString methods to SkImageFilter-derived classes

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index 890cb71b516b53c8611e0e34067bb503fcabb0fd..791d1fee1c0f496c74356fd5113fe44f97be2c07 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -16,6 +16,7 @@ public:
     SkAlphaThresholdFilterImpl(const SkRegion& region, SkScalar innerThreshold,
                                SkScalar outerThreshold, SkImageFilter* input);
 
+    SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkAlphaThresholdFilterImpl)
 
 protected:
@@ -374,3 +375,12 @@ bool SkAlphaThresholdFilterImpl::onFilterImage(Proxy*, const SkBitmap& src,
 
     return true;
 }
+
+#ifndef SK_IGNORE_TO_STRING
+void SkAlphaThresholdFilterImpl::toString(SkString* str) const {
+    str->appendf("SkAlphaThresholdImageFilter: (");
+    str->appendf("inner: %f outer: %f", fInnerThreshold, fOuterThreshold);
+    str->append(")");
+}
+#endif
+