allow SkBitmapCache to operate on a local instance, for testability

src/core/SkResourceCache.cpp

Index: src/core/SkResourceCache.cpp
diff --git a/src/core/SkResourceCache.cpp b/src/core/SkResourceCache.cpp
index 732557d0a55c14b6744621e6b37f90f663e257c3..3098a9a2a13cf678f21cd2144d07e5575fe4483b 100644
--- a/src/core/SkResourceCache.cpp
+++ b/src/core/SkResourceCache.cpp
@@ -432,6 +432,11 @@ size_t SkResourceCache::SetTotalByteLimit(size_t newLimit) {
     return get_cache()->setTotalByteLimit(newLimit);
 }
 
+SkResourceCache::DiscardableFactory SkResourceCache::GetDiscardableFactory() {
+    SkAutoMutexAcquire am(gMutex);
+    return get_cache()->discardableFactory();
+}
+
 SkBitmap::Allocator* SkResourceCache::GetAllocator() {
     SkAutoMutexAcquire am(gMutex);
     return get_cache()->allocator();