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

content/child/webcrypto/status.h

Index: content/child/webcrypto/status.h
diff --git a/content/child/webcrypto/status.h b/content/child/webcrypto/status.h
index e2220afa13b4f0da30e047be1ecb9c5dd27baffe..e4092410ccdc0a6d56b791508af3b36f79f4f44f 100644
--- a/content/child/webcrypto/status.h
+++ b/content/child/webcrypto/status.h
@@ -214,6 +214,9 @@ class CONTENT_EXPORT Status {
   // and algorithm.
   static Status ErrorCreateKeyBadUsages();
 
+  // No usages were specified when creating a key.

[eroman, 2014/11/21 23:35:34]

nit: "when creating a private or secret key" (since some public keys like ECDH
can have empty usages).

[nharper, 2014/11/22 00:26:45]

Done.

+  static Status ErrorCreateKeyEmptyUsages();
+
   // An EC key imported using SPKI/PKCS8 format had the wrong curve specified in
   // the key.
   static Status ErrorImportedEcKeyIncorrectCurve();