Auto merge of #20840 - fabricedesre:save-localstorage, r=Manishearth

Save local storage at every change and not just on shutdown

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Currently local storage is saved to `local_data.json` only at shutdown when the storage thread receives the `StorageThreadMsg::Exit` message. This is not reliable enough in any case, and especially problematic for a young engine like Servo. Data loss ensues...

I kept the exit message in the storage thread even if we are not doing work anymore. I can remove it if needed but it should be useful once we move to a better storage backend than a json file.

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