De-@mut pipeline
Prevent '&nbsp' from stripping as whitespace
Fix: whitespace is considered as spaces(U+0020), tabs(U+0009), and line breaks(U+000D U+000A)
auto merge of #1728 : june0cho/servo/whitespace_nbsp, r=SimonSapin

Fix #1727.
auto merge of #1726 : pcwalton/servo/shut-down-profiler, r=kmcallister

This fixes a hang in content tests in my deleafset branch. No idea why it worked before though…

This just replicates the code in non-headless mode over to the headless mode.

r? @kmcallister
De-@mut the FrameTree.
Update rust-layers submodule
auto merge of #1712 : larsbergstrom/servo/more_atmuts, r=larsbergstrom,metajack

These commits remove @mut from the rest of Servo except for the script crate.

They rely on the hack `Rc` type exported from rust-layers, which will be removed once that same version lands in our Rust upgrade, but is required for now because otherwise I'd have to make all of these structures either `Send` or `Freeze`.

r? @metajack or @kmcallister
Remove commented-out parts of Document.webidl and HTMLDocument.webidl.
auto merge of #1729 : Ms2ger/servo/clear-document-idl, r=jdm

I don't see a good reason to keep those around for now
Create a Line DisplayItem
auto merge of #1709 : sankha93/servo/issue221, r=larsbergstrom

This fixes #221.

I used the `BaseDisplayItem` class to define rectangle in which the line will be drawn. The width of the rectangle will be used as the thickness of the line segment.
layout: Run assign-widths in parallel.

5% layout speedup on mobile Wikipedia.
layout: Combine assign-widths and assign-heights traversals.

15% layout speedup on mobile Wikipedia.
layout: Make `ParallelPreorderFlowTraversal` more generic
layout: Construct flows in parallel, without a leaf set
layout: Create a font context in sequential mode