Fix SkOneShotDiscardablePixelRef to return discardable segment.

src/core/SkResourceCache.cpp

Index: src/core/SkResourceCache.cpp
diff --git a/src/core/SkResourceCache.cpp b/src/core/SkResourceCache.cpp
index 7d3c3420dba868c7ddf3eacd8d110cf1fb3a9774..46229fa5bdd987a77a05dff9e8b1964c345c57f9 100644
--- a/src/core/SkResourceCache.cpp
+++ b/src/core/SkResourceCache.cpp
@@ -87,6 +87,8 @@ protected:
     void onUnlockPixels() override;
     size_t getAllocatedSizeInBytes() const override;
 
+    SkDiscardableMemory* diagnostic_only_getDiscardable() const override { return fDM; }
+
 private:
     SkDiscardableMemory* fDM;
     size_t               fRB;