[WebSocket] Create an isolated copy of the KURL parameter in Bridge::Connect()

WorkerThreadableWebSocketChannel::Bridge::connect() creates a closure of
Peer::connect call from url. Since this closure will be used by the main
thread while this method is called from the worker thread, we need to
create an isolated copy of it.

Recent refactoring changed this method to use CallClosureTask::create +
bind from createCallbackTask. Since createCallbackTask wraps arguments
with CrossThreadCopier, not calling copy() on url was not a problem in
the old code.

On the other hand, in Bridge::initializ(), isolateCopy() call on
sourceURL is redundant since createCallbackTask is used there. This
change removes it.

BUG=351292

Review URL: https://codereview.chromium.org/196023005

git-svn-id: svn://svn.chromium.org/blink/trunk@168992 bbb929c8-8fbe-4397-9dbb-9b2b20218538