Document::create_collection's callback should return a boolean instead of a node. (#2027)
auto merge of #2031 : saneyuki/servo/2027, r=jdm

Fix #2027

@jdm
This change will decrease the generics for `Document::create_collection`.
Do you have any good idea to avoid this?

Or don't we have to consider it now?
style: Parse `min-height` and `max-height`
style: Implement `-servo-minimum-line-height`, a derived property from
`line-height`, and the infrastructure needed for derived properties to
work.

Eventually I would like this to be used for text decorations, like
WebKit does with `-webkit-text-decorations-in-effect`.
style: Parse `background-position` and `background-attachment`.
util: Add some utility methods to `SmallVec`.
util: Add a `rect_contains_point` utility method
servo: Implement stacking contexts and allow multiple layers per
pipeline. This handles fixed positioning mostly correctly.
layout: Take `min-height` and `max-height` into account
layout: Take padding into account for inline boxes
layout: Update `parallel.rs` to use `OpaqueNodeMethods`
layout: Support multiple boxes per node; don't store fixed/absolute
descendant links separately
layout: Implement `-servo-minimum-line-height`
layout: Rewrite the margin collapse code to work with negative margins.
layout: Implement enough of automatic table layout to pass Acid2.
layout: Implement pseudo-elements.
test: Add many reftests for parts of Acid2.
support: Update submodules.
layout: Address review feedback.
test: Fix some reftests.