[webcrypto] Match the error handling defined by the spec.

LayoutTests/crypto/exportKey.html

Index: LayoutTests/crypto/exportKey.html
diff --git a/LayoutTests/crypto/exportKey.html b/LayoutTests/crypto/exportKey.html
index 54c4caa19caa6bc1e68621a3779a0eac7f077445..cebc4eda27baef045083689e1aad57c52d70135b 100644
--- a/LayoutTests/crypto/exportKey.html
+++ b/LayoutTests/crypto/exportKey.html
@@ -13,7 +13,7 @@ description("Tests cypto.subtle.exportKey.");
 
 jsTestIsAsync = true;
 
-importTestKeys().then(function(result) {
+addTask(importTestKeys().then(function(result) {
     keys = result;
 
     // Invalid export formats.
@@ -26,10 +26,12 @@ importTestKeys().then(function(result) {
     shouldThrow("crypto.subtle.exportKey('raw', 3)");
 
     shouldBe("keys.aesCbcJustDecrypt.extractable", "false")
-    shouldThrow("crypto.subtle.exportKey('raw', keys.aesCbcJustDecrypt)");
+    shouldRejectPromiseWithNull("crypto.subtle.exportKey('raw', keys.aesCbcJustDecrypt)");
 
     // TODO(eroman): exportKey() is not implemented.
-}).then(finishJSTest, failAndFinishJSTest);
+}));
+
+completeTestWhenAllTasksDone();
 
 </script>