Auto merge of servo#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 servo#3289).

r? anyone

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webrender/src/render_task.rs

@@ -54,6 +54,7 @@ pub struct RenderTaskId {
     pub index: u32,
 
     #[cfg(debug_assertions)]
+    #[cfg_attr(feature = "replay", serde(default = "FrameId::first"))]
     frame_id: FrameId,
 }