[webcrypto] Make WebCryptoKey nullable.

Source/platform/exported/WebCryptoKey.cpp

Index: Source/platform/exported/WebCryptoKey.cpp
diff --git a/Source/platform/exported/WebCryptoKey.cpp b/Source/platform/exported/WebCryptoKey.cpp
index 85d4a77db9410fb636d56aed548a94685f593342..7d0646cd3291572e2b8fe417abd00b015ca367b5 100644
--- a/Source/platform/exported/WebCryptoKey.cpp
+++ b/Source/platform/exported/WebCryptoKey.cpp
@@ -63,31 +63,46 @@ WebCryptoKey WebCryptoKey::create(WebCryptoKeyHandle* handle, WebCryptoKeyType t
     return key;
 }
 
+WebCryptoKey WebCryptoKey::createNull()
+{
+    return WebCryptoKey();
+}
+
 WebCryptoKeyHandle* WebCryptoKey::handle() const
 {
+    ASSERT(!isNull());
     return m_private->handle.get();
 }
 
 WebCryptoKeyType WebCryptoKey::type() const
 {
+    ASSERT(!isNull());
     return m_private->type;
 }
 
 bool WebCryptoKey::extractable() const
 {
+    ASSERT(!isNull());
     return m_private->extractable;
 }
 
 const WebCryptoAlgorithm& WebCryptoKey::algorithm() const
 {
+    ASSERT(!isNull());
     return m_private->algorithm;
 }
 
 WebCryptoKeyUsageMask WebCryptoKey::usages() const
 {
+    ASSERT(!isNull());
     return m_private->usages;
 }
 
+bool WebCryptoKey::isNull() const
+{
+    return m_private.isNull();
+}
+
 void WebCryptoKey::assign(const WebCryptoKey& other)
 {
     m_private = other.m_private;