Remove some code duplication in the Compositor

It is possible to share the code which creates root layers.
M1456, Implement MIME sniffing initial Step
Sending exit message to devtools to exit.

Accepted and closed incoming streams

Added header to for constant use.

Removed extra spaces.

Corrected spacing to conform to coding standards.

Corrected spacing to conform to coding standards.

Corrected spacing to conform with coding standards.

Moving new import next to old import for devtools_traits as specified in comment.

Removed method definition to include as inline code and changed to iter_mut.

Using loops to exit devtools on server exit message and disconnected message to simply break as suggested in the comment.

Removing TODO comment for completed functionality.

Moved the operation for exit of devtools on servo closing outside the match loop.

Removing trailing ';' and adding new line before connection closing loop.
Fix wpt readme for mach changes
Use freetype-sys on Linux.

See:
https://github.com/servo/libfreetype2/pull/2
https://github.com/servo/skia/pull/38
https://github.com/servo/rust-azure/pull/114

Let’s close #3867, even though this only works around and doesn’t fix the underlying issue.
embedding: remove eutil::fptr_is_null, fix #3967, add Option<> to fptrs
List bzip2 as a requirement in README.md

… but not for Homebrew or MacPorts, OS X which apparently ships with it.
Use the correct Rust path in Android packaging
Disable the iframe content test due to #3986.
embedding: change Option<> unwrapping for fptrs to use match
embedding: convert callback Option<> matching to map()
gfx: Rewrite display list construction to make stacking-contexts more
first-class.

This implements the scheme described here:

    https://groups.google.com/forum/#!topic/mozilla.dev.servo/sZVPSfPVfkg

This commit changes Servo to generate one display list per stacking
context instead of one display list per layer. This is purely a
refactoring; there are no functional changes. Performance is essentially
the same as before. However, there should be numerous future benefits
that this is intended to allow for:

* It makes the code simpler to understand because the "new layer needed"
  vs. "no new layer needed" code paths are more consolidated.

* It makes it easy to support CSS properties that did not fit into our
  previous flat display list model (without unconditionally layerizing
  them):

  o `opacity` should be easy to support because the stacking context
    provides the higher-level grouping of display items to which opacity
    is to be applied.

  o `transform` can be easily supported because the stacking context
    provides a place to stash the transformation matrix. This has the side
    benefit of nicely separating the transformation matrix from the
    clipping regions.

* The `flatten` logic is now O(1) instead of O(n) and now only needs to
  be invoked for pseudo-stacking contexts (right now: just floats),
  instead of for every stacking context.

* Layers are now a proper tree instead of a flat list as far as layout
  is concerned, bringing us closer to a production-quality
  compositing/layers framework.

* This commit opens the door to incremental display list construction at
  the level of stacking contexts.

Future performance improvements could come from optimizing allocation of
display list items, and, of course, incremental display list
construction.
auto merge of #3984 : zmike/servo/embedding-option_callbacks, r=jdm

@jdm,  @larsbergstrom

#legday
Update web-platform-tests.
Fix binding generation for Callback Functions and Callback Interfaces
Allow passing arguments to setTimeout/setInterval callbacks
Mark vertical-lr-blocks.html as flaky to ease the pain while it's being investigated. #3926
auto merge of #3996 : jdm/servo/flakyvertical, r=Ms2ger

...investigated. #3926
auto merge of #3989 : jdm/servo/disableiframetest, r=mbrubeck
auto merge of #3766 : t29/servo/mime-sniffing, r=jdm

Issue: #3144 

We created a sniffer task in components/net/, added a call in resource_task load function to create a new sniffer task (sending all the data), and sniffer_task currently sends all the data back to resource_task.

The purpose of this request is to get feedback from @jdm on our progress before moving forward and writing tests.