No thread safety when passing ThreadableLoaderOptions from a worker thread

No thread safety when passing ThreadableLoaderOptions from a worker thread
https://bugs.webkit.org/show_bug.cgi?id=143790

Reviewed by Geoffrey Garen.

* loader/ThreadableLoader.h:
* loader/ThreadableLoader.cpp: (WebCore::ThreadableLoaderOptions::isolatedCopy): Added.

* loader/WorkerThreadableLoader.cpp:
(WebCore::WorkerThreadableLoader::MainThreadBridge::MainThreadBridge): Don't just send
a structure with strings to a different thread, that's bad.

* platform/CrossThreadCopier.h: I think that this is dead code, but for this bug,
just removing a clearly wrong specialization.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@182866 268f45cc-cd09-0410-ab3c-d52691b4dbfc