Use requestSecret on the client to request secrets

Rather than accessing private method to get the secret storage
object (this was a bit confusing when I grepped for 'requestSecret'
and didn't find anything).

spec/unit/crypto/verification/secret_request.spec.js

@@ -69,6 +69,7 @@ describe("self-verifications", () => {
                 _crossSigningInfo,
                 _secretStorage,
             },
+            requestSecret: _secretStorage.request.bind(_secretStorage),
             getUserId: () => userId,
         };