Makes GeneratePrivateData() public

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=229239892

media_cas_packager_sdk/public/wv_cas_ca_descriptor.h

@@ -63,8 +63,7 @@ class WvCasCaDescriptor {
   // where the descriptor will be added.
   virtual size_t CaDescriptorBaseSize() const;
 
- protected:
-  // Protected visibility to support unit testing.
+  // Return private data in the CA descriptor.
   virtual std::string GeneratePrivateData(const std::string& provider,
                                      const std::string& content_id) const;
 };