Remove support for {return,native,concrete}Type overrides in Bindings.conf.

We have no reason to support non-default type names, and this commit corrects
the computations for callbacks (which needed the override until now).
layout: Store containing block inline-size separately rather than
writing it to a temporary location and overwriting it.

This makes layout more idempotent, which is important for incremental
layout.

Also converts `is_root` to a set of flags and fixes a `TODO` concerning
percentage inline heights of images.
auto merge of #3632 : Ms2ger/servo/bindings-conf, r=Manishearth

We have no reason to support non-default type names, and this commit corrects
the computations for callbacks (which needed the override until now).
auto merge of #3631 : pcwalton/servo/idempotent-inline-size, r=glennw

This makes layout more idempotent, which is important for incremental
layout.

Also converts `is_root` to a set of flags and fixes a `TODO` concerning
percentage inline heights of images.

r? @glennw
Merge remote-tracking branch 'origin/master' into html4ever

Conflicts:
	components/layout/flow.rs