Bug 263615

The URL move constructor doesn't invalidate the "moved-out" URL. This can lead WebKit code to do weird things.

For example, URLKeepingBlobAlive contains a m_url data member and is often moved-out to pass to a lambda. The destructor of the "moved-out" URLKeepingBlobAlive then runs and calls `unregisterBlobURLHandleIfNecessary()`. `unregisterBlobURLHandleIfNecessary()` will try to use m_url after it's been moved out to see if the URL protocol is "blob". This  causes URL::protocolIs() to try to do out-of-bound access in the underlying String (since the URL is marked as valid, even though it's m_string was moved out and other data members that are indexes into that string were not reset). Luckily, String's operator[] just returns nil when doing an out of bounds access at the moment.