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, 2 insertions, 2 deletions

diff --git a/components/layout_2020/display_list/mod.rs b/components/layout_2020/display_list/mod.rs
index bfa8e9c266f..d4be0e26b71 100644
--- a/components/layout_2020/display_list/mod.rs
+++ b/components/layout_2020/display_list/mod.rs
@@ -5,12 +5,12 @@
 use std::cell::{OnceCell, RefCell};
 use std::sync::Arc;
 
+use base::id::BrowsingContextId;
+use base::WebRenderEpochToU16;
 use embedder_traits::Cursor;
 use euclid::{Point2D, SideOffsets2D, Size2D};
 use fnv::FnvHashMap;
 use gfx::text::glyph::GlyphStore;
-use gfx_traits::WebRenderEpochToU16;
-use msg::constellation_msg::BrowsingContextId;
 use net_traits::image_cache::UsePlaceholder;
 use script_traits::compositor::{CompositorDisplayListInfo, ScrollSensitivity, ScrollTreeNodeId};
 use servo_geometry::MaxRect;