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, 4 insertions, 3 deletions

diff --git a/components/shared/script_layout/wrapper_traits.rs b/components/shared/script_layout/wrapper_traits.rs
index 846943414ae..0adb51be244 100644
--- a/components/shared/script_layout/wrapper_traits.rs
+++ b/components/shared/script_layout/wrapper_traits.rs
@@ -9,9 +9,9 @@ use std::fmt::Debug;
 use std::sync::Arc as StdArc;
 
 use atomic_refcell::AtomicRef;
-use gfx_traits::{ByteIndex, FragmentType};
+use base::id::{BrowsingContextId, PipelineId};
+use gfx_traits::ByteIndex;
 use html5ever::{local_name, namespace_url, ns, LocalName, Namespace};
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use net_traits::image::base::{Image, ImageMetadata};
 use range::Range;
 use servo_arc::Arc;
@@ -25,7 +25,8 @@ use style::selector_parser::{PseudoElement, PseudoElementCascadeType, SelectorIm
 use style::stylist::RuleInclusion;
 
 use crate::{
-    GenericLayoutData, HTMLCanvasData, HTMLMediaData, LayoutNodeType, SVGSVGData, StyleData,
+    FragmentType, GenericLayoutData, HTMLCanvasData, HTMLMediaData, LayoutNodeType, SVGSVGData,
+    StyleData,
 };
 
 pub trait LayoutDataTrait: Default + Send + Sync + 'static {}