[webcryto] Validate key usages during key creation.

content/child/webcrypto/webcrypto_util.cc

Index: content/child/webcrypto/webcrypto_util.cc
diff --git a/content/child/webcrypto/webcrypto_util.cc b/content/child/webcrypto/webcrypto_util.cc
index c6bf4b20a826ca1894cbcb3d3220fb3cead1a29d..47d5b21dc58fee319ffca7e8ce9ebcfdbde55817 100644
--- a/content/child/webcrypto/webcrypto_util.cc
+++ b/content/child/webcrypto/webcrypto_util.cc
@@ -179,6 +179,11 @@ bool CreateSecretKeyAlgorithm(const blink::WebCryptoAlgorithm& algorithm,
   }
 }
 
+bool ContainsKeyUsages(blink::WebCryptoKeyUsageMask a,
+                       blink::WebCryptoKeyUsageMask b) {
+  return (a & b) == b;
+}
+
 }  // namespace webcrypto
 
 }  // namespace content