Auto merge of #14344 - asajeffrey:script-thread-new-layout-use-incomplete-loads, r=jdm

Script thread creating layout thread should use the incomplete loads

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When a script thread creates a new layout thread, it does so by sending a message to an existing layout thread asking it to spawn. At the moment, we're only looking at the completed loads for that layout thread, so we can get a panic if two loads happen in quick succession. The temporary fix is to look for the layout thread in the incomplete loads too.

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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 #14333.
- [X] These changes do not require tests because it fixes a panic.

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