Check that usage isn't empty when generateKey() is called

content/child/webcrypto/nss/sym_key_nss.cc

Index: content/child/webcrypto/nss/sym_key_nss.cc
diff --git a/content/child/webcrypto/nss/sym_key_nss.cc b/content/child/webcrypto/nss/sym_key_nss.cc
index 5066d5d91d276bffcdb953d8ae412659518f18c8..d555787cc1a02f27d2c2cc32d0313823ec3ae677 100644
--- a/content/child/webcrypto/nss/sym_key_nss.cc
+++ b/content/child/webcrypto/nss/sym_key_nss.cc
@@ -43,6 +43,11 @@ Status GenerateSecretKeyNss(const blink::WebCryptoKeyAlgorithm& algorithm,
   if (!key_data)
     return Status::OperationError();
 
+  // TODO(nharper): mask usages with allowed usages

[eroman, 2014/11/20 23:40:10]

Testing the allowed usages is the responsibility of the caller of
GenerateSecretKeyNss, so I don't think this TODO is necessary.

[nharper, 2014/11/21 22:12:01]

Done.

+  if (usages == 0) {

[eroman, 2014/11/20 23:40:09]

The style being used in these files is that single line if statements omit
braces. Please match it for new code.

[nharper, 2014/11/21 22:12:01]

Done.

+    return Status::ErrorCreateKeyBadUsages();

[eroman, 2014/11/20 23:40:10]

Introduce a new error message so this is easier to understand for web
developers.

Perhaps:
 Status::ErrorGenerateKeyEmptyUsages()

[nharper, 2014/11/21 22:12:01]

Done.

+  }
+
   scoped_ptr<SymKeyNss> handle(new SymKeyNss(
       pk11_key.Pass(), CryptoData(key_data->data, key_data->len)));