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/net/tests/http_loader.rs b/components/net/tests/http_loader.rs
index 324b75daa7f..1db85d44ecc 100644
--- a/components/net/tests/http_loader.rs
+++ b/components/net/tests/http_loader.rs
@@ -11,6 +11,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex, RwLock};
 use std::time::Duration;
 
+use base::id::TEST_PIPELINE_ID;
 use cookie_rs::Cookie as CookiePair;
 use crossbeam_channel::{unbounded, Receiver};
 use devtools_traits::{
@@ -29,7 +30,6 @@ use http::{Method, StatusCode};
 use hyper::{Body, Request as HyperRequest, Response as HyperResponse};
 use ipc_channel::ipc;
 use ipc_channel::router::ROUTER;
-use msg::constellation_msg::TEST_PIPELINE_ID;
 use net::cookie::Cookie;
 use net::cookie_storage::CookieStorage;
 use net::http_loader::determine_requests_referrer;