Implement canComputeFastBounds() for image filters.

src/core/SkImageFilter.cpp

Index: src/core/SkImageFilter.cpp
diff --git a/src/core/SkImageFilter.cpp b/src/core/SkImageFilter.cpp
index 3214eeb40eee8c3ecf4d61113b726f1f1722a58f..5f9fa64110dbb171932759cce6c36ea3ffb2730c 100644
--- a/src/core/SkImageFilter.cpp
+++ b/src/core/SkImageFilter.cpp
@@ -284,6 +284,23 @@ void SkImageFilter::computeFastBounds(const SkRect& src, SkRect* dst) const {
     }
 }
 
+bool SkImageFilter::canComputeFastBounds() const {
+    if (this->affectsTransparentBlack()) {
+        return false;
+    }
+    for (int i = 0; i < fInputCount; i++) {
+        SkImageFilter* input = this->getInput(i);
+        if (input && !input->canComputeFastBounds()) {
+            return false;
+        }
+    }
+    return true;
+}
+
+bool SkImageFilter::affectsTransparentBlack() const {
+    return false;
+}
+
 bool SkImageFilter::onFilterImage(Proxy*, const SkBitmap&, const Context&,
                                   SkBitmap*, SkIPoint*) const {
     return false;