allow SkBitmapCache to operate on a local instance, for testability

BUG=skia:
R=mtklein@google.com, danakj@chromium.org, piotaixr@chromium.org, junov@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/576763002
diff --git a/src/core/SkResourceCache.cpp b/src/core/SkResourceCache.cpp
index 732557d..3098a9a 100644
--- a/src/core/SkResourceCache.cpp
+++ b/src/core/SkResourceCache.cpp
@@ -432,6 +432,11 @@
     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();