[webcrypto] Make WebCryptoKey nullable.

Source/core/platform/chromium/support/WebCrypto.cpp

Index: Source/core/platform/chromium/support/WebCrypto.cpp
diff --git a/Source/core/platform/chromium/support/WebCrypto.cpp b/Source/core/platform/chromium/support/WebCrypto.cpp
index f2d4bc9c5d5e5e0a73be7b09f837d97d930357d8..0d58262ec00e7bf57e41b1174ad6ad1499d7cbf4 100644
--- a/Source/core/platform/chromium/support/WebCrypto.cpp
+++ b/Source/core/platform/chromium/support/WebCrypto.cpp
@@ -66,12 +66,15 @@ void WebCryptoResult::completeWithBoolean(bool b)
 
 void WebCryptoResult::completeWithKey(const WebCryptoKey& key)
 {
+    ASSERT(!key.isNull());
     m_impl->completeWithKey(key);
     reset();
 }
 
 void WebCryptoResult::completeWithKeyPair(const WebCryptoKey& publicKey, const WebCryptoKey& privateKey)
 {
+    ASSERT(!publicKey.isNull());
+    ASSERT(!privateKey.isNull());
     m_impl->completeWithKeyPair(publicKey, privateKey);
     reset();
 }