obs-browser: Fix a deadlock
This fixes a freeze that can occur if you try to destroy browser while
another browser is being created. The CEF UI thread has to wait on a
window message to the main application UI thread, meanwhile the destroy
call in the main application UI thread is waiting on the CEF UI thread,
thus causes a deadlock.
Now that we have the SetParent(hwnd, nullptr) code that detaches the CEF
window from the Qt window, we no longer have to worry about
synchronously shutting down the browser, so instead of waiting for that
operation to finish, just allow it to occur asynchronously.
jp9000