Auto merge of #12860 - Coder206:iterWarn, r=Wafflespeanut

Fix iter_arith warning when running ./mach build-cef

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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #12855 (github issue number if applicable).

- [x] There are tests for these changes (./mach build-cef)

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ports/cef/lib.rs

@@ -6,7 +6,6 @@
 #![feature(box_syntax)]
 #![feature(core_intrinsics)]
 #![feature(filling_drop)]
-#![feature(iter_arith)]
 #![feature(link_args)]
 #![feature(plugin)]
 #![feature(unicode)]