trigger an update via the built-in software update mechanism.

Reviewed by Sam Weinig.

* WebKitLauncher/WebKitLauncher.xcodeproj/project.pbxproj:
* WebKitLauncher/WebKitLauncherURLProtocol.h: Added.
* WebKitLauncher/WebKitLauncherURLProtocol.m: Added.
(+[WebKitLauncherURLProtocol load]):
(+[WebKitLauncherURLProtocol canInitWithRequest:]): Only allow use of the x-webkit-launcher scheme from .webkit.org subdomains.
(+[WebKitLauncherURLProtocol canonicalRequestForRequest:]):
(-[WebKitLauncherURLProtocol startLoading]):
(-[WebKitLauncherURLProtocol stopLoading]):
(-[WebKitLauncherURLProtocol handleIsWebKitLauncherAvailableJS]): Return a brief JavaScript snippet that can be used to programatically
determine whether the x-webkit-launcher is available and working.
(-[WebKitLauncherURLProtocol handleCheckForUpdates]): Trigger a software update on the main thread.
(-[WebKitLauncherURLProtocol resourceNotFound]): Fail with a generic "File does not exist" error.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@44920 268f45cc-cd09-0410-ab3c-d52691b4dbfc