Make ComputedStyle fields private and add getters.

This isolates layout code from upcoming refactoring in properties.rs.mako.
Un-disable and fix some warnings in the style crate.
Remove CowArc and use the new Arc::make_unique() method instead.
Remove unnecessary pubs from script_task
auto merge of #2478 : SimonSapin/servo/style-getters, r=metajack

This isolates layout code from upcoming refactoring in properties.rs.mako.
Make some scary things in the workqueue module private
auto merge of #2499 : mbrubeck/servo/unpub, r=metajack
Make the workqueue's type parameter names less terrible
Don't overwrite the "Host" request header

Fixes broken requests caused by #2493.
auto merge of #2500 : bjz/servo/workqueue, r=pcwalton

cc. @pcwalton
Prefix fragment types in inline.rs with `Inline`

This is to reduce confusion once we rename `servo::box_::Box` to `servo::fragment::Fragment`.
Convert usages of `Box` in type identifiers to `Fragment`
Rename functions, fields and adjust comments to use `fragment` instead of `box`

I tried to make sure to keep 'box' where it made sense, and alter comments so they still make sense.
Rename `box_.rs` to `fragment.rs`
Improve naming of structs in inline.rs
Improve line index documentation
Update submodules for EXT_DEPS dependency tracking.
auto merge of #2501 : mbrubeck/servo/host, r=pcwalton

Fixes broken requests caused by #2493.
auto merge of #2497 : bjz/servo/rename-box-type, r=pcwalton

I tried to make sure to keep 'box' where it made sense, and alter comments so they still make sense.

r? @pcwalton
Don't implement `ne` for ImageResponseMsg, since it matches the default.