Hide webdriverCallback and webdriverTimeout methods behind a pref

[jdm]

servo/components/script/dom/webidls/Window.webidl

Lines 144 to 149 in de01770

	// WebDriver extensions
	partial interface Window {
	// Shouldn't be public, but just to make things work for now
	void webdriverCallback(optional any result);
	void webdriverTimeout();
	};

This pref can be set for all tests in https://github.com/web-platform-tests/wpt/blob/708893d8180931603d2e6b5e74c45fc2954c4b18/tools/wptrunner/wptrunner/executors/executorservo.py#L43.

[jdm]

One other subtlety - these will need to be accessible when running with --webdriver, so we should actually enable the pref automatically when the server is started.