Auto merge of #11695 - asajeffrey:script-thread-dont-panic, r=Ms2ger

Removed some sources of panic from script thread.

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**This PR needs some thought!**

It removes some sources of panic from script_thread.rs, caused by pipeline lookup failure. It's incomplete, as there are some uses of `get_browsing_context` elsewhere in the code.

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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 #11693 and #11685 (and probably some other intermittents)
- [X] These changes do not require tests because it is fixing intermittent panic

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