Auto merge of #5361 - luniv:css-device-adapt, r=mbrubeck

Spec: http://dev.w3.org/csswg/css-device-adapt/

Currently, the actual viewport is used by the layout task as part of the reflow, and the compositor uses the zoom constraints. I'm not sure if anywhere else currently needs access to the constraints (i.e. there's no CSSOM as far as I can tell).

I did not implement sections 9 (viewport <META>) or 10 (handling 'auto' for 'zoom').

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components/msg/constellation_msg.rs

@@ -14,6 +14,7 @@ use layers::geometry::DevicePixel;
 use util::cursor::Cursor;
 use util::geometry::{PagePx, ViewportPx};
 use std::sync::mpsc::{channel, Sender, Receiver};
+use style::viewport::ViewportConstraints;
 use webdriver_traits::WebDriverScriptCommand;
 use url::Url;
 
@@ -233,7 +234,9 @@ pub enum Msg {
     /// Requests that the constellation retrieve the current contents of the clipboard
     GetClipboardContents(Sender<String>),
     // Dispatch a webdriver command
-    WebDriverCommand(PipelineId, WebDriverScriptCommand)
+    WebDriverCommand(PipelineId, WebDriverScriptCommand),
+    /// Notifies the constellation that the viewport has been constrained in some manner
+    ViewportConstrained(PipelineId, ViewportConstraints),
 }
 
 #[derive(Clone, Eq, PartialEq)]

components/msg/lib.rs

@@ -9,6 +9,7 @@ extern crate hyper;
 extern crate layers;
 extern crate util;
 extern crate url;
+extern crate style;
 extern crate webdriver_traits;
 
 #[cfg(target_os="macos")]