[webcrypto] Make all of the crypto.subtle method failures asynchronous.

LayoutTests/crypto/sign-verify.html

Index: LayoutTests/crypto/sign-verify.html
diff --git a/LayoutTests/crypto/sign-verify.html b/LayoutTests/crypto/sign-verify.html
index e7fded06a26944448233e5de043090ce2fee0641..8e4fcc1801027274d7f7d18c8fe6d8e726c98806 100644
--- a/LayoutTests/crypto/sign-verify.html
+++ b/LayoutTests/crypto/sign-verify.html
@@ -131,11 +131,11 @@ allTests.push(importTestKeys().then(function(importedKeys) {
     shouldThrow("crypto.subtle.verify(hmacSha1, keys.hmacSha1, [], data)");
 
     // Operation does not support signing.
-    shouldThrow("crypto.subtle.sign({name: 'sha-1'}, keys.hmacSha1, data)");
+    shouldRejectPromiseWithNull("crypto.subtle.sign({name: 'sha-1'}, keys.hmacSha1, data)");
 
     // Operation doesn't support signing (also given an invalid key, but the
     // first failure takes priority)
-    shouldThrow("crypto.subtle.sign({name: 'RSAES-PKCS1-v1_5'}, keys.hmacSha1, data)");
+    shouldRejectPromiseWithNull("crypto.subtle.sign({name: 'RSAES-PKCS1-v1_5'}, keys.hmacSha1, data)");
 
     // Key's algorithm must match.
     shouldRejectPromiseWithNull("crypto.subtle.sign({name: 'hmac', hash: {name: 'sha-256'}}, keys.hmacSha1, data)");