Updating built-in externs

I think it would be nice to review and update built-in externs https://github.com/google/closure-compiler/tree/master/externs/browser . It seems to me, that browser/ie_dom, browser/ie_event and some others correspond to the DOM model of old browsers and they are not used anymore. There usually is a W3C alternative for the same thing. It would allow minifying even more properties in the user's code and maybe could make the compilation process faster.

The structure of externs is strange, too. browser/gecko_dom contains "selectionStart" property, which was [defined in W3C](https://www.w3.org/TR/html5/forms.html#dom-textarea/input-selectionstart) and is not gecko-specific.