Add a command line flag --v8-proxy-resolver, to select the new PAC implementation.
When running in single process mode, this flag has no effect (since we can't run side by side with the renderer's V8).
In regular mode, the v8 resolver is currently running in the browser process.
This means it has to share with the v8 debugger shell.
Added locking around the debugger shell so they can peacefully co-exist.
When this flag is enabled, PAC scripts are downloaded through the browser.