DiscIO: Add IsCached virtual function to BlobReader.

Source/Core/DiscIO/Blob.h

@@ -97,6 +97,9 @@ public:
     return false;
   }
 
+  // Returns true only for CachedBlobReader.
+  virtual bool IsCached() const { return false; }
+
 protected:
   BlobReader() {}
 };