Web Inspector: fix closure compiler warnings in extension server and API
https://bugs.webkit.org/show_bug.cgi?id=109563

Reviewed by Vsevolod Vlasov.

* inspector/front-end/ExtensionAPI.js: drive-by: make sure we fail if extensionServer is not defined in outer scope.
* inspector/front-end/ExtensionServer.js:
(WebInspector.ExtensionServer.prototype.):
(WebInspector.ExtensionServer.prototype._onEvaluateOnInspectedPage):
* inspector/front-end/externs.js: add extensionServer

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142603 268f45cc-cd09-0410-ab3c-d52691b4dbfc