servo: Merge #18317 - Enable Custom Elements by Default (from cbrewster:custom_elements_enable); r=jdm

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #9372 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because enabling pref flag.

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Source-Repo: https://github.com/servo/servo
Source-Revision: e10e56930fa8733e47f0a201aa5a80a5dc7c2d10

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : daf04f94252c04ecacc3915292d7a96632d79d88

servo/resources/prefs.json

@@ -3,7 +3,7 @@
   "dom.bluetooth.testing.enabled": false,
   "dom.canvas-text.enabled": false,
   "dom.compositionevent.enabled": false,
-  "dom.customelements.enabled": false,
+  "dom.customelements.enabled": true,
   "dom.forcetouch.enabled": false,
   "dom.gamepad.enabled": false,
   "dom.mouseevent.which.enabled": false,