devtools: Fix getBreakpointListActor handler in watcher actor (#38624)

the Firefox devtools client often sends multiple
`getBreakpointListActor` requests to the watcher actor when connecting,
and the Firefox devtools server returns the same breakpoint list actor
every time, but Servo returns a new breakpoint list actor each time.
this patch aligns Servo’s behaviour with Firefox.

Testing: this patch adds a devtools test

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: atbrakhi <atbrakhi@igalia.com>

components/devtools/actors/breakpoint.rs

@@ -52,4 +52,8 @@ impl BreakpointListActor {
     pub fn new(name: String) -> Self {
         Self { name }
     }
+
+    pub fn encodable(&self) -> BreakpointListActorMsg {
+        BreakpointListActorMsg { actor: self.name() }
+    }
 }