[webcrypto] Make it safe to delete WebCryptoResult from any thread.

public/platform/WebCrypto.h

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 121 matching lines @@
     //
     // The result can be set either synchronously while handling the request,
     // or asynchronously after the method has returned. When completing
     // asynchronously make a copy of the WebCryptoResult and call it from the
     // same thread that started the request.
     //
     // -----------------------
     // Threading
     // -----------------------
     //
-    // The WebCrypto interface will only be called from the render's main
-    // thread. All communication back to Blink must be on this same thread.
+    // The WebCrypto interface will be called from blink threads (main or
+    // web worker). All communication back to Blink must be on this same thread.
+    //
     // Notably:
     //
-    //   * The WebCryptoResult is NOT threadsafe. It should only be used from
-    //     the Blink main thread.
+    //   * The WebCryptoResult can be copied between threads, however all
+    //     methods other than the destructor must be called from the origin
+    //     Blink thread.
     //
     //   * WebCryptoKey and WebCryptoAlgorithm ARE threadsafe. They can be
     //     safely copied between threads and accessed. Copying is cheap because
     //     they are internally reference counted.
     //
     //   * WebArrayBuffer is NOT threadsafe. It should only be created from the
-    //     Blink main thread. This means threaded implementations may have to
+    //     target Blink thread. This means threaded implementations may have to
     //     make a copy of the output buffer.
     //
     // -----------------------
     // Inputs
     // -----------------------
     //
     //   * Data buffers are passed as (basePointer, byteLength) pairs.
     //     These buffers are only valid during the call itself. Asynchronous
     //     implementations wishing to access it after the function returns
     //     should make a copy.
@@ skipping 89 matching lines @@
     // Returns true on success.
     virtual bool serializeKeyForClone(const WebCryptoKey&, WebVector<unsigned char>&) { return false; }
 
 protected:
     virtual ~WebCrypto() { }
 };
 
 } // namespace blink
 
 #endif