Implement canComputeFastBounds() for image filters.

src/effects/SkRectShaderImageFilter.cpp

Index: src/effects/SkRectShaderImageFilter.cpp
diff --git a/src/effects/SkRectShaderImageFilter.cpp b/src/effects/SkRectShaderImageFilter.cpp
index cdf03131c1cd89f1e0cf2ec2f97aaef9f4cf5983..45962537c8244f70b5c6c59277a22be560afbb5d 100644
--- a/src/effects/SkRectShaderImageFilter.cpp
+++ b/src/effects/SkRectShaderImageFilter.cpp
@@ -79,6 +79,10 @@ bool SkRectShaderImageFilter::onFilterImage(Proxy* proxy,
     return true;
 }
 
+bool SkRectShaderImageFilter::affectsTransparentBlack() const {
+    return true;
+}
+
 #ifndef SK_IGNORE_TO_STRING
 void SkRectShaderImageFilter::toString(SkString* str) const {
     str->appendf("SkRectShaderImageFilter: (");