servo: Merge #18885 - Add a default 'unstable' feature to CEF (from servo:unstable-cef); r=nox

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.

Source-Repo: https://github.com/servo/servo
Source-Revision: 086c48210c1ea0b65c91e1b300f796758080871f

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 140f57c4860727e94c65cee232f16d0c14128a8e

servo/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"}