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, 3 insertions, 5 deletions

diff --git a/components/shared/compositing/constellation_msg.rs b/components/shared/compositing/constellation_msg.rs
index 2a64db84651..2536a6cfc05 100644
--- a/components/shared/compositing/constellation_msg.rs
+++ b/components/shared/compositing/constellation_msg.rs
@@ -6,16 +6,14 @@ use std::collections::HashMap;
 use std::fmt;
 use std::time::Duration;
 
+use base::id::{BrowsingContextId, PipelineId, TopLevelBrowsingContextId, WebViewId};
+use base::Epoch;
 use embedder_traits::Cursor;
-use gfx_traits::Epoch;
 use ipc_channel::ipc::IpcSender;
 use keyboard_types::KeyboardEvent;
-use msg::constellation_msg::{
-    BrowsingContextId, PipelineId, TopLevelBrowsingContextId, TraversalDirection, WebViewId,
-};
 use script_traits::{
     AnimationTickType, CompositorEvent, GamepadEvent, LogEntry, MediaSessionActionType,
-    WebDriverCommandMsg, WindowSizeData, WindowSizeType,
+    TraversalDirection, WebDriverCommandMsg, WindowSizeData, WindowSizeType,
 };
 use servo_url::ServoUrl;