This prevents concurrent access to the same state from different
worker threads. We needed to treat windowState specially because we
explicitly want it to last beyond the life-span of a single context.
So we store it as a static variable in `native.mm` and synchronize
access with a static `NSLock`. Good enough for now.
We send a message from the render process to the browser process indicating which path to open. Also start introducing a `v8_extensions::` namespace to contain the extensions.
