Implement a persistent uniqueID-based cache for SkImageFilter.

src/core/SkBitmapDevice.cpp

Index: src/core/SkBitmapDevice.cpp
diff --git a/src/core/SkBitmapDevice.cpp b/src/core/SkBitmapDevice.cpp
index 09b3b605f85a3e4b36a28e77d88a75135abfae3b..8c92f71dceee7aacee32d684293abab260b937ba 100644
--- a/src/core/SkBitmapDevice.cpp
+++ b/src/core/SkBitmapDevice.cpp
@@ -367,6 +367,12 @@ const void* SkBitmapDevice::peekPixels(SkImageInfo* info, size_t* rowBytes) {
     return NULL;
 }
 
+SkImageFilter::UniqueIDCache* SkBitmapDevice::getImageFilterCache() {
+    SkImageFilter::UniqueIDCache* cache = SkImageFilter::UniqueIDCache::Get();
+    cache->ref();
+    return cache;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 bool SkBitmapDevice::filterTextFlags(const SkPaint& paint, TextFlags* flags) {