Disable ViewportMetrics unless MouseEvent lacks support for pageX/pageY

For #1300, only IE8 lacks `pageX`/`pageY` (and perhaps really old FF or w/e I guess).

- [ ] Test plan: not yet (want your input on the PR first)
- [ ] If approved: will need to make a manual test and verify in all browsers

From my https://github.com/facebook/react/issues/1300#issuecomment-52509117:

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https://github.com/facebook/react/blob/master/src/browser/ui/ReactEventListener.js#L65
https://github.com/facebook/react/blob/master/src/browser/ui/ReactEventListener.js#L165

That makes no sense at all to me, `refresh` does not take an argument, yet we get the scroll position and provide it as an argument. Instead, `refresh` gets the scroll position internally and uses that? So we call `getUnboundedScrollPosition` twice, but only use the result once (also, this should be broken for events inside iframes, for IE8).

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https://github.com/facebook/react/blob/master/src/browser/eventPlugins/TapEventPlugin.js#L54
I'm pretty sure any browsers that supports touch also supports `pageX`/`pageY`... ?
I think @petehunt's plan was that we'd only access the DOM in files in src/browser/ui/ – but maybe ReactBrowserEventEmitter should be moved there?