Auto merge of #3339 - servo:kvark-frame-id, r=emilio
Auto merge of #3339 - servo:kvark-frame-id, r=emilio

Add default deserializer for frame_id

Since frame ID is debug-only, it's not serialized when capturing from a release-built Firefox, making us unable to replay it with debug Wrench. This PR fixes this (broken recently by #3289).

r? anyone

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