Auto merge of #13785 - gterzian:use_veqdequeu_in_servoparser, r=nox

switch to using DOMRefCell<VeqDeque<String>> for ServoParser::pending…

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The field ServoParser::pending_input should be a DOMRefCell<VeqDeque<String>>, this lets us use VecDeque::pop_front instead of Vec::remove in ServoParser::take_next_input_chunk.

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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 #13690 (github issue number if applicable).

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…_input

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