Copy the downloaded file of ResourceResponse to the other threads

To leverage the downloadToFile option in XHR, copying downloadedFile is
needed since XHR can work in the worker threads.
To copy the File to the other threads, we take the similar aproach to
RedirectToFileResourceHandler and SerializedScriptValue. Cloning the
internal Blob URL to keep the life of the File and reconstitute a File
in the other threads from the path, this internal URL and the type.

Currently the downloadToFile interface is not exposed to the script, so
we cannot test it. However, when XHR uses the downloadToFile option,
this change will be tested by LayoutTests.

BUG=230560, 269055

Review URL: https://chromiumcodereview.appspot.com/23610003

git-svn-id: svn://svn.chromium.org/blink/trunk@157011 bbb929c8-8fbe-4397-9dbb-9b2b20218538