Remove old rendering backend.

This removes paint threads, rust-layers dependency, and changes
optional webrender types to be required.

The use_webrender option has been removed, however I've left
the "-w" command line option in place so that wpt
runner can continue to pass that. Once it's removed from there
we can also remove the -w option.

Once this stage is complete, it should be fine to change the
display list building code to generate webrender display
lists directly and avoid the conversion step.

components/script_layout_interface/Cargo.toml

@@ -31,4 +31,3 @@ selectors = "0.13"
 string_cache = {version = "0.2.26", features = ["heap_size"]}
 style = {path = "../style"}
 url = {version = "1.2", features = ["heap_size"]}
-util = {path = "../util"}

components/script_layout_interface/lib.rs

@@ -39,7 +39,6 @@ extern crate selectors;
 extern crate string_cache;
 extern crate style;
 extern crate url;
-extern crate util;
 
 pub mod message;
 pub mod reporter;

components/script_layout_interface/message.rs

@@ -21,7 +21,6 @@ use style::context::ReflowGoal;
 use style::selector_impl::PseudoElement;
 use style::stylesheets::Stylesheet;
 use url::Url;
-use util::ipc::OptionalOpaqueIpcSender;
 
 /// Asynchronous messages that script can send to layout.
 pub enum Msg {
@@ -148,7 +147,6 @@ pub struct NewLayoutThreadInfo {
     pub constellation_chan: IpcSender<ConstellationMsg>,
     pub script_chan: IpcSender<ConstellationControlMsg>,
     pub image_cache_thread: ImageCacheThread,
-    pub paint_chan: OptionalOpaqueIpcSender,
     pub content_process_shutdown_chan: IpcSender<()>,
     pub layout_threads: usize,
 }