Bug 1539213 - Emit `Target.targetCreated` for already-opened tabs. r=ato

When `Target.setDiscoverTargets` is called, one `Target.targetCreated` event
should be sent per already opened tab.

Differential Revision: https://phabricator.services.mozilla.com/D25558

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

remote/domains/parent/Target.jsm

@@ -32,6 +32,9 @@ class Target extends Domain {
     } else {
       targets.off("connect", this.onTargetCreated);
     }
+    for (const target of targets) {
+      this.onTargetCreated("connect", target);
+    }
   }
 
   onTargetCreated(eventName, target) {

remote/test/browser/browser_target.js

@@ -42,9 +42,15 @@ async function testCDP() {
   const {Target} = client;
   ok("Target" in client, "Target domain is available");
 
+  const targetCreatedForAlreadyOpenedTab = Target.targetCreated();
+
   // Instruct the server to fire Target.targetCreated events
   Target.setDiscoverTargets({ discover: true });
 
+  // Calling `setDiscoverTargets` will dispatch `targetCreated` event for all
+  // already opened tabs
+  await targetCreatedForAlreadyOpenedTab;
+
   // Create a new target so that the test runs against a fresh new tab
   const targetCreated = Target.targetCreated();
   const {targetId} = await Target.createTarget();