Remove unneeded code now that image libraries use Vec<T>.
Move the computed style to a new SharedLayoutData struct.

Implementing the CSSOM requires giving the script task access to the
computed style for a node. Moving it into a new SharedLayoutData struct
member of LayoutDataRef seems to be the best way to achieve this.

This is the first step towards #1721.
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.
Remove the argcAndIndex argument from instantiateJSToNativeConversionTemplate.

This commit does not change the generated code.
Remove the isOptional argument and the dealWithOptional return value from getJSToNativeConversionTemplate.

Note that dictionaries currently don't support members without default values
right now.

This commit does not change the generated code.
auto merge of #2393 : Ms2ger/servo/initialize_js_info, r=jdm

There is no particular leason to initilaize js_info lazily.
Remove unused and misnamed url field on HTMLIFrameElement.
auto merge of #2401 : Ms2ger/servo/iframe-url, r=jdm
Drop the pointless indirection in Window::active_timers.
Stop using ~[u8] in http_loader::load.
Avoid unnecessary allocations in try_parse_url.
auto merge of #2405 : Ms2ger/servo/active_timers, r=jdm
auto merge of #2406 : Ms2ger/servo/http_loader-load, r=jdm
auto merge of #2407 : Ms2ger/servo/try_parse_url-owned, r=jdm