Add window.crypto.subtle behind a runtime feature flag ("Crypto")

Source/modules/crypto/Crypto.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 52 matching lines @@
         ec = TYPE_MISMATCH_ERR;
         return;
     }
     if (array->byteLength() > 65536) {
         ec = QUOTA_EXCEEDED_ERR;
         return;
     }
     cryptographicallyRandomValues(array->baseAddress(), array->byteLength());
 }
 
+SubtleCrypto* Crypto::subtle()
+{
+    if (!m_subtleCrypto)
+        m_subtleCrypto = SubtleCrypto::create();
+    return m_subtleCrypto.get();
 }
+
+}