Bug 1506549 - correctly type workerListChanged on ContentProcessTarget spec. r=yulia

It allows to send the event through the front rather than DebuggerClient.

MozReview-Commit-ID: H8zEwAlUWDb

Depends on D11622

Differential Revision: https://phabricator.services.mozilla.com/D11623

--HG--
extra : moz-landing-system : lando

devtools/shared/specs/targets/content-process.js

@@ -27,6 +27,9 @@ const contentProcessTargetSpec = generateActorSpec({
       type: "newSource",
       source: Option(0, "json"),
     },
+    workerListChanged: {
+      type: "workerListChanged",
+    },
   },
 });