Fix caching bug in new SkImageFilter::filterImage path

src/core/SkImageFilter.cpp

Index: src/core/SkImageFilter.cpp
diff --git a/src/core/SkImageFilter.cpp b/src/core/SkImageFilter.cpp
index a233871bff69465171f64764702dc4d36a92c7dc..374a561030254561941ecf21e67d4a65cdfffe80 100644
--- a/src/core/SkImageFilter.cpp
+++ b/src/core/SkImageFilter.cpp
@@ -231,6 +231,8 @@ SkSpecialImage* SkImageFilter::filterImage(SkSpecialImage* src, const Context& c
     SkSpecialImage* result = this->onFilterImage(src, context, offset);
     if (result && context.cache()) {
         context.cache()->set(key, result, *offset);
+        SkAutoMutexAcquire mutex(fMutex);
+        fCacheKeys.push_back(key);
     }
 
     return result;