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.

obs-browser @ 8d77568b

-Subproject commit d4c835c344c3a0bf1c1edb94ad4f7ec509f29a66
+Subproject commit 8d77568bf4f5d98b097cdd555be397914271af80