Fix a deprecated phase warning.
Remove some unused fields.
Mark some unused fields.
Remove some deprecated into_owned calls.
Merge pull request #2762 from Ms2ger/warnings

Fix some build warnings. r=me
Update to latest rust-layers
Merge pull request #2755 from mrobinson/remove-common

Update to latest rust-layers
Fix 100-column violations in compositor.rs
Replace bare constructor functions with 'new' static member functions in rust-azure.
Fix encoding issues for getResponseHeader()
Update expectations for #2765
Merge pull request #2760 from zwarich/compositor-100-columns

Fix 100-column violations in compositor.rs
Merge pull request #2761 from zwarich/remove-set-transform

Stop calling Scene::set_transform
Add a ShutdownState type in the compositor

Currently the compositor tracks its state in the processing of shutting
down with two boolean instance variables `shutting_down` and `done`, but
these two variables really implement a three-state abstraction.
Merge pull request #2765 from Manishearth/xhr-wpt-encoding

Fix encoding issues for getResponseHeader()
Merge pull request #2763 from Ms2ger/azure-ctors

Replace bare constructor functions with 'new' static member functions in rust-azure; r=SimonSapin
Implement Element.matches().
Made Page.window_size and Page.next_subpage_id Traceable, added trait Encodable for WindowSizeData, ViewportPx, PagePx, and DevicePixel
Next stage of refactoring font system. This commit introduces
the font cache task, and adapts client code to use it. It also
cleans up some existing code paths.

- Fonts are only read once from disk while in use (they
  are discarded if the reference count reaches zero, however).
  This saves memory and prevents unnecessary reading from disk.
- It will be easier to add web font support, as all fonts
  are created and managed in a single place and the entire
  pipeline ensures that only one in-memory copy of font data
  is required.

An overview of how the pieces fit together:

FontTemplate - A structure containing everything that
is required to create (and select) font handles. This
structure is shared among all matching font handles
(via Arc).

FontTemplateData - A platform specific structure that
contains the actual font data inside a template (this is
a byte array on Linux/Android, CTFont on Mac).

FontHandle - An opaque, platform specific handle to
a font instance. Each FontHandle contains an Arc<>
reference to the FontTemplate it was created from.

FontCache - This is a separate task, that is responsible
for loading and caching FontTemplate structures. There
is one FontCache per constellation. It is only ever accessed
via the FontContext described below.

FontContext - This is the public interface to the FontCache
and is used by the layout and render code to create font
handles. These must *not* be shared between threads. There
is typically one FontContext per thread/task.
Warning police.