Add a default 'unstable' feature to CEF

I recently moved that default from libservo to ports/servo,
which made CEF not have it anymore.

The different defaults made `./mach build-cef` after `./mach build`
rebuild many libraries (including serde and all of its recursive dependents)
and take a long time.

ports/cef/Cargo.toml

@@ -16,6 +16,9 @@ opt-level = 3
 
 [features]
 debugmozjs = ["libservo/debugmozjs"]
+default = ["unstable", "default-except-unstable"]
+default-except-unstable = []
+unstable = ["libservo/unstable"]
 
 [dependencies]
 compositing = {path = "../../components/compositing"}