Remove unneeded code now that image libraries use Vec<T>.
auto merge of #2387 : glennw/servo/image-fixups, r=Ms2ger
Convert as many of Page's RefCell fields to Cell as possible (fixes #1990).
auto merge of #2390 : Ms2ger/servo/1990-Page-RefCell, r=jdm
Add UIEvent::new_uninitialized().
Add MouseEvent::new_uninitialized().
auto merge of #2384 : saneyuki/servo/2383, r=Ms2ger

Fix #2383

These changes introduce `FooEvent::new_uninitialized()` constructor. This constructor use to create `FooEvent` without calling `FooEvent::InitFooEvent`.

#2383 says integrating `FooEvent::new()` and `InitFooEvent`, but we need to preserve simple `FooEvent::new_uninitialized()` for `document.createEvent()`.

@jdm r?
Remove ScriptTask::initialize_js_info.

There is no particular leason to initilaize js_info lazily.
auto merge of #2393 : Ms2ger/servo/initialize_js_info, r=jdm

There is no particular leason to initilaize js_info lazily.