Move non-gfx things out of `gfx_traits` and create a `base` crate (#32296)

For a long time, `gfx_traits` has held a lot of things unrelated to graphics
and also unrelated to the `gfx` crate (which is mostly about fonts).
This is a cleanup which does a few things:

1. Move non `gfx` crate things out of `gfx_traits`. This is important in
   order to prevent dependency cycles with a different integration between
   layout, script, and fonts.
2. Rename the `msg` crate to `base`. It didn't really contain anything
   to do with messages and instead mostly holds ids, which are used
   across many different crates in Servo. This new crate will hold the
   *rare* data types that are widely used.

Details:

 - All BackgroundHangMonitor-related things from base to a new
   `background_hang_monitor_api` crate.
 - Moved `TraversalDirection` to `script_traits`
 - Moved `Epoch`-related things from `gfx_traits` to `base`.
 - Moved `PrintTree` to base. This should be widely useful in Servo.
 - Moved `WebrenderApi` from `base` to `webrender_traits` and renamed it
   to `WebRenderFontApi`.

1 files changed, 1 insertions, 1 deletions

diff --git a/components/script/layout_dom/node.rs b/components/script/layout_dom/node.rs
index 0cbdba64573..7a521d36220 100644
--- a/components/script/layout_dom/node.rs
+++ b/components/script/layout_dom/node.rs
@@ -8,9 +8,9 @@ use std::borrow::Cow;
 use std::fmt;
 use std::sync::Arc as StdArc;
 
+use base::id::{BrowsingContextId, PipelineId};
 use gfx_traits::ByteIndex;
 use html5ever::{local_name, namespace_url, ns};
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use net_traits::image::base::{Image, ImageMetadata};
 use range::Range;
 use script_layout_interface::wrapper_traits::{