Returns fake preferences if PrefValue::Missing.

If the pref_map().get() enum returns PrefValue::Missing, then
fake values are returned just like before.

components/devtools/actors/preference.rs

@@ -67,11 +67,50 @@ impl Actor for PreferenceActor {
                 let _ = stream.write_json_packet(&reply);
                 ActorMessageStatus::Processed
             },
-            PrefValue::Missing => ActorMessageStatus::Ignored,
+            PrefValue::Missing => handle_missing_preference(self.name(), msg_type, stream),
         })
     }
 }
 
+// if the preferences are missing from pref_map then we return a
+// fake preference response based on msg_type.
+fn handle_missing_preference(
+    name: String,
+    msg_type: &str,
+    stream: &mut TcpStream,
+) -> ActorMessageStatus {
+    match msg_type {
+        "getBoolPref" => {
+            let reply = BoolReply {
+                from: name,
+                value: false,
+            };
+            let _ = stream.write_json_packet(&reply);
+            ActorMessageStatus::Processed
+        },
+
+        "getCharPref" => {
+            let reply = CharReply {
+                from: name,
+                value: "".to_owned(),
+            };
+            let _ = stream.write_json_packet(&reply);
+            ActorMessageStatus::Processed
+        },
+
+        "getIntPref" => {
+            let reply = IntReply {
+                from: name,
+                value: 0,
+            };
+            let _ = stream.write_json_packet(&reply);
+            ActorMessageStatus::Processed
+        },
+
+        _ => ActorMessageStatus::Ignored,
+    }
+}
+
 #[derive(Serialize)]
 struct BoolReply {
     from: String,