[dom] Consider making `timestamp` a DOMHighResTimestamp (#80)

https://dom.spec.whatwg.org/#dom-event-timestamp

Tracking discussion from elsewhere (eg. [here](https://bugzilla.mozilla.org/show_bug.cgi?id=77992#c43)) about the plan to try to change `Event.timestamp` into a `DOMHighResTimestamp`.  In addition to changing the type, what other spec changes are needed?

To me the main value we'd get from `timestamp` is if it's permitted to carry information about when logically the action occurred (which may be before the "event was created").  This can be used, for example, to get some measure of input event processing latency.  WebKit has [done this for input events for several years](https://bugs.webkit.org/show_bug.cgi?id=117179) and we've been debating about how to expose this in blink/chromium [even longer](https://code.google.com/p/chromium/issues/detail?id=160524).

@birtles @majido @tdresser

If we find it's not compatible to make these changes to `timestamp` then we should still take everything we've learned here and apply it to a new field, eg. `platformTimestamp'.

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