DOM should add hooks for sync events that happen during insert/remove operations

[travisleithead]

As discsussed at Custom Elements F2F:

• blur
• focus
• beforeunload
  (other?)
  ...events require some synchronous processing during tree insertion and/or removal
  (This may have been discussed during the part of the meeting where there was nothing scribed...?)

[travisleithead]

@annevk, I have been working with Edge devs on re-working our new DOM to handle synchronous callouts during tree mutation APIs (specifically for handling mutation events). We came up with some invariants that might be worth considering for the spec:

1. Tree operation primitives cannot fail (e.g., they must always succeed in the end)
2. 'Remove' tree primitive guarantees that on completion the target node has no parent.
3. 'Insert' tree primitive can assume that the target node has no parent.

[annevk]

Gecko bug on blur (doesn't fire it at the moment): https://bugzilla.mozilla.org/show_bug.cgi?id=559561

@esprehn will try firing these events using custom element callback timing in Blink.

[annevk]

See also #305.